fix(uploads): finish the sweeps round 1 left half-done

Deep-review round 2, on round 1's own fix commit. It found MORE than round 1
did (28 raw, against 13), and most of it was mine -- a new piece of UI attracts
findings a backend-only diff never does. The pattern in almost all of them is
the same: a principle applied to the site that prompted it and not to its
siblings.

* `display_text` reached three of the FIVE writers of a user-supplied name.
  `POST /api/downloads` and `POST /api/downloads/edit` still handed Postgres an
  unbounded string, so a 300-character name pasted into the Add-download dialog
  or the editor was a bare 500 (main.py re-raises every DataError but the NUL
  case). Four of the seven reviewers found it independently. The bound now lives
  in `models.py`, the three columns are DECLARED with it, and the guard for the
  next writer is parsed rather than remembered: every `display_name` /
  `display_uploader` assignment under `app/routes/` must go through the helper.
  That guard immediately found two more -- the Google sign-in callback writes
  the profile name into the same 255-wide column -- so it is six, not five.

* The connection release reached one of the THREE routes that read their own
  body. The screenshot route (10 MiB, from a phone) and the client log sink both
  still held a pooled connection idle-in-transaction for the length of the
  upload, which is the invariant db.py sizes the pool on. All three now call
  `db.release_read_connection`, whose precondition -- nothing pending on the
  shared session -- is CHECKED rather than described, because a whole-session
  rollback discarding a caller's staged write is a mistake this repo has already
  made once. `test_body_limit.py` holds the rule for the next such route.

* The AST drift registry keyed its sites by (module, limit expression), so a
  second route copied from the first collapsed onto the same entry and went
  uncovered -- the exact hole the registry exists to close. Keyed per call site
  now, and it carries the release requirement too.

* The dialog's own refusal was announced once and never again: two over-cap
  picks leave an identical node with identical text, and an untouched live
  region says nothing. It is keyed per pick now, the drop zone reads as invalid
  and points at the sentence, and the reason is reachable from the control
  rather than only from a one-shot announcement.

* Two tests rebuilt the blind spot they were written to remove: the dialog's cap
  test sat nowhere near the boundary (a `>` loosened to `>=` survived it, which
  is the mutation round 1 fixed on the server side), and the header-precedence
  test only covered an exact-case duplicate while a differently-cased one would
  have been COMBINED onto the wire rather than replaced. Both fixed, and the
  merge now dedupes case-insensitively because header names are.

Also: the upload dialog's unstubbed usage query was firing a real request from
jsdom in a neighbouring suite and leaking a delayed "connection lost" toast
across test boundaries; `GET /api/downloads/usage` had no test at all though the
whole client-side check hangs off it; and the comment claiming the edge 413 is
readable now says what was measured -- readable to 200 MiB over loopback, a bare
`onerror` through a proxy -- so the pre-send check is named as the mechanism and
that branch as the backstop it is.

Every test added or changed here was mutation-checked. One mutation SURVIVED and
is recorded rather than papered over: the `tooBig` term inside `submit()` cannot
be reached, because `ui/Button` swallows the activation itself, so it is marked
as belt and braces instead of being described as the guard.

frontend/src/components/UploadDialog.dom.test.tsx

@@ -84,30 +84,79 @@ describe("UploadDialog", () => {
     expect(btn.disabled).toBe(true);
   });
 
-  it("refuses an over-cap file here, naming the limit, instead of sending it", async () => {
+  // `file.size > cap`, on the boundary in both directions — the cap is inclusive server-side
+  // (backend test_upload.py pins that a body of EXACTLY the cap is accepted), so a dialog that
+  // refused it would make a legal file un-uploadable with no way round. Two sizes either side of
+  // one cap, because a single over-cap case passes just as happily with `>` loosened to `>=`.
+  const CAP = 8;
+  const pickFile = (bytes: number, name = "f.bin") =>
+    fireEvent.change(fileInput(), {
+      target: { files: [new File(["x".repeat(bytes)], name)] },
+    });
+
+  it("refuses a file one byte over the cap, naming the limit, instead of sending it", async () => {
     // Both server-side caps answer only after the transfer — and on a default install the one that
-    // answers is the edge ceiling, whose refusal carries no number at all. So the dialog checks the
-    // size itself: the user learns the limit before a byte moves.
-    h.downloadUsage.mockResolvedValue({ upload_max_bytes: 4 });
+    // answers is the edge ceiling, whose refusal carries no number at all and may not even reach
+    // the browser. So the dialog checks the size itself: the user learns the limit before a byte
+    // moves.
+    h.downloadUsage.mockResolvedValue({ upload_max_bytes: CAP });
     wrap();
     await waitFor(() => expect(h.downloadUsage).toHaveBeenCalled());
-    fireEvent.change(fileInput(), {
-      target: { files: [new File(["far too many bytes"], "big.bin")] },
-    });
+    pickFile(CAP + 1, "big.bin");
 
     await waitFor(() => expect(screen.getByText("downloads.errors.upload.tooLarge")).toBeTruthy());
     const btn = screen.getByText("downloads.upload.submit").closest("button")!;
     expect(btn.disabled).toBe(true);
+    // The drop zone carries the reason, because a natively disabled Submit is not in the tab order
+    // to be asked for it.
+    const zone = screen.getByText("big.bin").closest("button")!;
+    expect(zone.getAttribute("aria-invalid")).toBe("true");
+    expect(zone.getAttribute("aria-describedby")).toBe(screen.getByRole("alert").id);
+  });
+
+  it("swallows a Submit press that arrives while the button still holds focus", async () => {
+    // The OTHER half of "never sent": the test above covers the natively disabled button (not
+    // focused), this one covers the soft-disabled one. `ui/Button` is focus-safe, so a button that
+    // turns unavailable while it HOLDS focus stays in the tab order and stays clickable, and it is
+    // the primitive's own click guard that refuses — measured: deleting the `tooBig` term from
+    // `submit()` changes nothing here, which is why that term is documented as belt and braces
+    // rather than as the mechanism. What this pins is the user-visible contract: pressing Upload
+    // after dropping a bigger file does not start a transfer the server will refuse.
+    h.downloadUsage.mockResolvedValue({ upload_max_bytes: CAP });
+    wrap();
+    await waitFor(() => expect(h.downloadUsage).toHaveBeenCalled());
+    pickFile(CAP, "fits.bin");
+    const btn = screen.getByText("downloads.upload.submit").closest("button")!;
+    btn.focus();
+    fireEvent.focus(btn);
+    pickFile(CAP + 1, "toobig.bin");
+    await waitFor(() => expect(screen.getByRole("alert")).toBeTruthy());
+    expect(btn.disabled).toBe(false); // soft-disabled, still focusable — and still clickable
     fireEvent.click(btn);
     expect(h.uploadDownload).not.toHaveBeenCalled();
   });
 
-  it("sends a file that fits, so the check above is not simply refusing everything", async () => {
-    h.downloadUsage.mockResolvedValue({ upload_max_bytes: 1_000_000 });
+  it("sends a file of EXACTLY the cap, which the server accepts", async () => {
+    h.downloadUsage.mockResolvedValue({ upload_max_bytes: CAP });
     wrap();
     await waitFor(() => expect(h.downloadUsage).toHaveBeenCalled());
-    fireEvent.change(fileInput(), { target: { files: [new File(["small"], "ok.bin")] } });
+    pickFile(CAP, "exact.bin");
+    expect(screen.queryByRole("alert")).toBeNull();
     fireEvent.click(screen.getByText("downloads.upload.submit"));
     await waitFor(() => expect(h.uploadDownload).toHaveBeenCalledTimes(1));
   });
+
+  it("re-announces the refusal when a SECOND over-cap file is picked", async () => {
+    // Failing the same way again has to be announced again. The message names the CAP, not the
+    // file, so two over-cap picks leave the node and its text identical — and a live region that
+    // is never touched says nothing. See formAlerts.dom.test.tsx, which holds the same rule for
+    // the three other alerts in this app that can repeat.
+    h.downloadUsage.mockResolvedValue({ upload_max_bytes: CAP });
+    wrap();
+    await waitFor(() => expect(h.downloadUsage).toHaveBeenCalled());
+    pickFile(CAP + 1, "a.bin");
+    const first = await screen.findByRole("alert");
+    pickFile(CAP + 2, "b.bin");
+    await waitFor(() => expect(screen.getByRole("alert")).not.toBe(first));
+  });
 });