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/routes/admin_logs.py

@@ -26,10 +26,11 @@ from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy import Text, and_, cast, func, or_, select
 from sqlalchemy.orm import Session
 
+from app import logsink
 from app.auth import admin_user
 from app.db import get_db
 from app.models import LogEvent, User
-from app.utils import iso, like_escape
+from app.utils import like_escape
 
 router = APIRouter(prefix="/api/admin/logs", tags=["admin"])
 
@@ -66,25 +67,6 @@ def _decode_cursor(cursor: str) -> tuple[datetime, int]:
         raise HTTPException(status_code=400, detail={"code": "bad_cursor"}) from exc
 
 
-def _serialize(row: LogEvent) -> dict:
-    return {
-        "id": row.id,
-        "ts": iso(row.ts),
-        "level": row.level,
-        "source": row.source,
-        "logger": row.logger,
-        "event": row.event,
-        "msg": row.msg,
-        "attrs": row.attrs,
-        "exc": row.exc,
-        "request_id": row.request_id,
-        "user_id": row.user_id,
-        "job_id": row.job_id,
-        "debug_session_id": row.debug_session_id,
-        "version": row.version,
-    }
-
-
 def _accounts(db: Session, user_ids: set[int]) -> dict[str, str]:
     """The accounts a page names, by id → address. An id with no account left (erased after the
     line was written) is simply absent, and the client shows the bare id."""
@@ -196,7 +178,7 @@ def list_logs(
     more = len(rows) > limit
     rows = rows[:limit]
     return {
-        "items": [_serialize(r) for r in rows],
+        "items": [logsink.serialize(r) for r in rows],
         "accounts": _accounts(db, {r.user_id for r in rows if r.user_id is not None}),
         "next_cursor": _encode_cursor(rows[-1]) if more else None,
     }