refactor(capture): the unit's tables move into downloads/models

The nine Capture tables (files, jobs, folders, shares, links, quota, cookies, profiles, library
resume points) leave app/models.py for the unit's own module; app.tables registers every unit's
tables for the migrations, the test database and the ownership check. No schema change.

frontend/src/lib/api/downloads.contract.test.ts

@@ -29,8 +29,10 @@ describe("display-name bound", () => {
   it("is the width the columns are actually declared with", () => {
     // The mutation guard for the check above: both constants could agree on a number neither
     // column uses. `String(DISPLAY_TEXT_MAX)` is how the three display columns are declared, so a
-    // literal creeping back into one of them fails here.
-    const models = src("../../../../backend/app/models.py");
+    // literal creeping back into one of them fails here. `User.display_name` is a core table; the
+    // job's two are Capture's.
+    const models =
+      src("../../../../backend/app/models.py") + src("../../../../backend/app/downloads/models.py");
     const declared = [...models.matchAll(/display_(?:name|uploader): Mapped.*?String\((.*?)\)/g)];
     expect(declared).toHaveLength(3);
     for (const m of declared) expect(m[1]).toBe("DISPLAY_TEXT_MAX");