feat: Settings → Diagnostics, a consented log level and the tab's own log

A user can raise their diagnostics level to Részletes (DEBUG) or Teljes (TRACE) for 24 hours from a
new Settings tab, after a consent box that says what is recorded, that the server keeps it and for
how long (the instance's own retention), and that it lapses. Alap is the base and needs no consent.

- users.diagnostics_level/_until (migration 0087), written only by PUT /api/me/diagnostics, which
  computes the expiry itself; the demo account cannot raise it. /api/me reports the level in force.
- The server logs an elevated user's requests at that level and stores every line of them
  (logs.LevelGate on the stdout handler and the sink; the root is lowered only while an elevation
  is active). Teljes adds the redacted query string to the request line.
- POST /api/diagnostics/events stores the browser's log as source=web rows: account from the
  session, body capped while streaming, events/attrs capped, re-redacted, events under the
  consented level dropped, timestamps clamped, 30 batches a minute, 409 once the level lapsed.
- lib/diagnostics.ts applies the level to the tab and streams only what happens after consent, in
  batches, never its own request lines, with a keepalive flush on pagehide; stops on lapse (its
  own timer), on withdrawal and on 409.
- Detail hooks at DEBUG: which control was clicked (test id, icon, app copy in English — never user
  text, input values or a link's query), visibility/bfcache/resize, filter changes
  (lib/filterLog.ts; typed search only at TRACE), player events (lib/mediaLog.ts; media errors are
  warnings at every level). Offline/online at the base level.
- "Saját napló" viewer: live, newest first, level groups with counts, search, row details, copy
  the filtered events as JSON lines, clear.
- Privacy policy and Terms exist in Hungarian and English (a contract test holds both languages to
  the same sections); the policy now describes the server logs and diagnostic logging, self-service
  deletion and password sign-in.

Fixed on the way:
- A sync auth dependency ran logs.set_user on a copied context, so the account never reached a
  handler's own log lines (measured); a per-request mutable scope carries it now.
- The Settings tab rail left a phone no room (clipped labels, buttons over text): below `sm` it is
  a scrolling row above the content.
- PUT /api/me/preferences took a bare dict body; it is a Pydantic model now.

frontend/src/lib/log.ts

@@ -4,8 +4,9 @@
 // the same fields and level names as a server line, so a client batch can land in `log_events` beside
 // the server's own rows and a request joins across the two by `request_id`.
 //
-// Events are kept in memory only, in a bounded ring: nothing leaves the browser from here. (Sending a
-// batch is the diagnostics switch's job, and only with the user's consent — E22 S5.)
+// Events are kept in memory only, in a bounded ring: nothing leaves the browser from here. Sending
+// them is `lib/diagnostics.ts`'s job, and only while the user's consented level is raised (E22 S5);
+// it and the Settings log viewer follow the ring through `subscribe`.
 //
 // REDACTION HAPPENS HERE, at capture, not at the call site — the same rules as the server's handler,
 // held to them by a fixture both test suites run (`backend/tests/fixtures/log_redaction_cases.json`).
@@ -144,9 +145,34 @@ export function events(): LogEvent[] {
   return ring.slice();
 }
 
-/** Drop every kept event (tests; later the viewer's "clear"). */
+/** Drop every kept event (the viewer's "clear", tests). */
 export function clear(): void {
   ring = [];
+  notify(null);
+}
+
+// Told about every kept event as it is kept (`null` = the ring was cleared). A listener that throws
+// is its own problem: it must not stop the others, nor the code that logged.
+type Listener = (event: LogEvent | null) => void;
+const listeners = new Set<Listener>();
+
+/** Follow the ring: called with each event once it is kept, and with `null` when it is cleared.
+ *  Returns the unsubscribe. */
+export function subscribe(listener: Listener): () => void {
+  listeners.add(listener);
+  return () => {
+    listeners.delete(listener);
+  };
+}
+
+function notify(event: LogEvent | null): void {
+  for (const listener of listeners) {
+    try {
+      listener(event);
+    } catch {
+      // see Listener
+    }
+  }
 }
 
 function emit(
@@ -172,6 +198,7 @@ function emit(
     if (requestId) entry.request_id = requestId;
     ring.push(entry);
     if (ring.length > RING_MAX) ring.splice(0, ring.length - RING_MAX);
+    notify(entry);
   } catch {
     // Logging must never break the code that logs (an attr with a throwing getter, a cycle).
   }