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/sysconfig.py

@@ -150,6 +150,11 @@ SPECS: tuple[ConfigSpec, ...] = (
     # --- Audit log ---
     # Days to keep admin audit-log rows before the audit_gc job prunes them (0 = keep forever).
     ConfigSpec("audit_retention_days", "int", "audit", "audit_retention_days", min=0, max=3650),
+    # --- Diagnostics (E22) ---
+    # Runtime log level for the API process: TRACE | DEBUG | INFO | WARNING | ERROR. Free-text like
+    # search_source (an unknown value falls back to INFO in logs.resolve_level, so a typo can't
+    # silence the app). Applied without a restart — see routes/config.py and main.py's lifespan.
+    ConfigSpec("log_level", "str", "diagnostics", "log_level"),
     # --- Plex integration (optional; local-file playback + Plex metadata) ---
     ConfigSpec("plex_enabled", "bool", "plex", "plex_enabled"),
     ConfigSpec("plex_server_url", "str", "plex", "plex_server_url"),