docs: refresh 7 stale module headers that describe shipped work as "later phase" (E3 S3, B-core-12)

This codebase leans on "why" comments as its architecture doc, so a header that
claims the opposite of the code below it is worse than none. Fix the seven the
review flagged: channels.py (subscribe/unsubscribe shipped), playlists.py +
sync/playlists.py (YouTube two-way sync + write-back shipped), setup.py (drop the
"epic 6c" ref — the wizard steps exist), config.py ("auth._client_ip" ->
"routes.auth._client_ip"), db.py ("routes/plex.py" -> "routes/plex/" after the
package split), sysconfig.py (the "Phase C" note — the watch-sync job that reads
plex_watch_sync_interval_min has landed; clarified it's a boot default, not a knob).
Docs only.

backend/app/routes/setup.py

@@ -3,7 +3,7 @@
 `GET /api/setup/status` is public (the SPA uses it to decide whether to show the wizard). Every
 mutating step goes through `require_setup`: it rejects once the instance is configured AND requires
 the one-time setup token (printed to the container logs at first boot, carried by the wizard as the
-X-Setup-Token header). The wizard steps themselves are added in epic 6c.
+X-Setup-Token header).
 """
 from typing import Any