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/sync/playlists.py

@@ -1,6 +1,6 @@
-"""Read-direction YouTube playlist sync: mirror each user's own YouTube playlists into
-local `source='youtube'` playlists (kept fresh by the scheduler). One-way (YT -> local);
-the write-back direction (local edits -> YouTube) is a later phase.
+"""YouTube playlist sync, both directions. Read: mirror each user's own YouTube playlists
+into local `source='youtube'` playlists (kept fresh by the scheduler). Write-back: push local
+edits out to YouTube (`plan_push` / `push_playlist`), behind the optional write scope.
 
 YouTube's special Watch Later / History playlists are not exposed by the Data API and are
 never synced. Videos in a playlist that aren't in our shared catalog yet are fetched and