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.

docker-compose.selfhost.yml

@@ -10,6 +10,15 @@
 # Everything else — the admin account, Google sign-in, SMTP — is set in the web wizard on
 # first run. See docs/self-hosting.md.
 
+# Bounded container logs. Docker's json-file driver keeps every line forever by default, and the API
+# now writes one structured line per request (E22 S2) — 3×50 MB per service is days of history on a
+# busy instance and a hard ceiling on the disk it can take.
+x-logging: &logging
+  driver: json-file
+  options:
+    max-size: "50m"
+    max-file: "3"
+
 services:
   db:
     image: postgres:16-alpine
@@ -27,6 +36,7 @@ services:
       interval: 10s
       timeout: 5s
       retries: 12
+    logging: *logging
     restart: unless-stopped
 
   api:
@@ -71,6 +81,7 @@ services:
       timeout: 5s
       retries: 5
       start_period: 30s
+    logging: *logging
     restart: unless-stopped
 
   # Download worker: same image, runs the yt-dlp / ffmpeg job loop instead of the API. Shares the
@@ -109,6 +120,7 @@ services:
       - ALL
     tmpfs:
       - /tmp
+    logging: *logging
     restart: unless-stopped
 
   # PO-token provider sidecar: mints YouTube Proof-of-Origin tokens so the worker beats bot-detection
@@ -119,6 +131,7 @@ services:
     networks: [internal]
     security_opt:
       - no-new-privileges:true
+    logging: *logging
     restart: unless-stopped
 
 volumes: