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/db.py

@@ -12,7 +12,7 @@ class Base(DeclarativeBase):
 
 # Pool sized above the sync-endpoint threadpool (~40) so a burst of short DB sessions can't starve on
 # checkout. The image proxies deliberately DON'T hold a connection during their slow external fetch
-# (see routes/plex.py), so this headroom is only for the many brief sessions, not long ones.
+# (see routes/plex/), so this headroom is only for the many brief sessions, not long ones.
 engine = create_engine(
     settings.database_url,
     pool_pre_ping=True,