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/downloads/runner.py

@@ -17,6 +17,7 @@ import shutil
 import subprocess
 import sys
 
+from app import logs
 from app.config import settings
 from app.db import SessionLocal
 from app.downloads import edit as editmod
@@ -52,12 +53,15 @@ def _deprioritize() -> None:
 
 
 def main() -> None:
-    logging.basicConfig(level=logging.INFO)
+    logs.configure_logging("runner")
     _deprioritize()
     if len(sys.argv) != 2:
         print("usage: python -m app.downloads.runner <job_id>", file=sys.stderr)
         raise SystemExit(2)
     job_id = int(sys.argv[1])
+    # This whole process exists to run ONE job — tag every line it prints with it, so the child's
+    # output joins up with the worker's and the API's view of the same download.
+    logs.set_job(job_id)
 
     with SessionLocal() as db:
         job = db.get(DownloadJob, job_id)