feat(diagnostics): debug sessions — start, stop, the tag on every line, screenshots (E16 S1)

A session is one recording, capped at 60 minutes by the server. While it is
open, a request naming the caller's own session in X-Debug-Session is logged
at the session's level with every line tagged; a header naming anything else
is ignored. The browser's batches count a session as consent to its level and
keep the tag only on its own events. Screenshots are judged by their bytes,
capped in size and count, and served back to their owner only.

- debug_sessions / debug_screenshots tables, one open session per user
  (partial unique index), log_events.debug_session_id indexed
- /api/debug-sessions: start (joins a running one), stop, list, get, events,
  screenshots; /api/me reports the live session
- the hourly log job closes expired sessions and drops them with the logs
- logsink.serialize and routes._common.capped_body shared by their readers

backend/app/auth.py

@@ -4,7 +4,7 @@ from sqlalchemy import delete, func, select
 from sqlalchemy.orm import Session
 from starlette.requests import HTTPConnection
 
-from app import diagnostics
+from app import debugsession
 from app import email as email_mod
 from app import logs
 from app import mailtext
@@ -367,7 +367,11 @@ def _session_user(request: Request, db: Session, user_id: int | None, is_default
     # session middleware and this task's context. `request.state` is shared across middleware layers
     # (it lives on the ASGI scope), so the access-log line can read it back. The user's consented
     # diagnostics level rides along: their requests are logged — and stored — in that detail (S5).
-    logs.set_user(user.id, diagnostics.effective_level(user), user.diagnostics_until)
+    # So does a debug session the request names (E16): only the caller's own open one counts, and a
+    # header naming anything else is ignored rather than refused — a stale tab must keep working.
+    session = debugsession.from_header(db, user, request.headers.get(debugsession.HEADER))
+    level, until = debugsession.level_for(user, session)
+    logs.set_user(user.id, level, until, session.id if session else None)
     request.state.user_id = user.id
     return user