feat: one structured log for every process, with a request id

A deployment printed four different log formats and none of them could be
joined up: uvicorn's log_config.json for the API, a hand-rolled StreamHandler in
main.py for anything not started by uvicorn, logging.basicConfig() in the worker
and in the download child (no timestamp at all), and alembic's fileConfig.
app/logs.py replaces all four with one JSON line per event (OpenTelemetry's log
data model, no SDK): ts, level, severity, source, logger, event code, message,
attributes, plus the correlation the context carries — request id, account,
download job.

Redaction is applied at the HANDLER, not at the call sites: every message, every
attribute and every traceback is scrubbed on the way out, so a secret cannot
leak by being forgotten at one of ~125 call sites. Passwords, tokens, cookies,
secret config values and email addresses never reach a line.

The access log is ours now (`http.request`): the route template instead of the
concrete path, the status, the duration and the account — and never the query
string, which uvicorn's access log printed on every single request (`?account=`,
`?source=`, the `?token=` of a share link). A well-formed client request id is
accepted and echoed in X-Request-Id, so a browser line and a server line join up.
The account comes from `request.state`, filled by the auth dependency: the
middleware is the outermost one, so it runs outside SessionMiddleware and in a
different task context.

`log_level` becomes an admin knob (sysconfig), applied without a restart and
re-applied at startup — best-effort on purpose, since a logging knob must never
be what stops the app from booting. Every compose service gets bounded json-file
logs (3×50 MB): Docker keeps every line forever by default, and the API now
writes one line per request.

Verified on the localdev stack: JSON everywhere including alembic and the
worker, the query strings gone, a supplied X-Request-Id echoed and logged, and
three /healthz hits of which only the one made while the level was DEBUG
produced a line.

backend/app/routes/config.py

@@ -10,6 +10,7 @@ from sqlalchemy.orm import Session
 
 from app import audit
 from app import email as email_mod
+from app import logs
 from app import mailtext
 from app import sysconfig
 from app.audit import AuditAction
@@ -29,6 +30,14 @@ def _redact(v):
     return _URL_USERINFO.sub(r"\1***@", v) if isinstance(v, str) else v
 
 
+def _apply_runtime(db: Session, key: str) -> None:
+    """Some knobs change how THIS process behaves, not just what a later read returns — they have to
+    take effect now, without a restart. One dispatch for both call sites (set and reset), so the next
+    such key can't be wired into one of them and forgotten in the other."""
+    if key == "log_level":
+        logs.set_level(sysconfig.get_str(db, "log_level"))
+
+
 @router.get("")
 def get_config(_: User = Depends(admin_user), db: Session = Depends(get_db)) -> dict:
     return sysconfig.describe(db)
@@ -80,6 +89,7 @@ def set_config(
             after=None if s.secret else {"value": _redact(new)},
         )
         db.commit()
+    _apply_runtime(db, key)
     return sysconfig.describe(db)
 
 
@@ -104,6 +114,7 @@ def reset_config(
         before=None if s.secret else {"value": _redact(old)},
     )
     db.commit()
+    _apply_runtime(db, key)
     return sysconfig.describe(db)