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/sysconfig.py

@@ -96,8 +96,8 @@ SPECS: tuple[ConfigSpec, ...] = (
     ConfigSpec("plex_overlay_map", "str", "plex", "plex_overlay_map"),
     # (plex_sync_interval_min is tuned from the Scheduler dashboard, not here — it's a scheduler job.)
     ConfigSpec("plex_max_transcodes", "int", "plex", "plex_max_transcodes", min=0, max=16),
-    # NOTE: plex_watch_sync_interval_min (config.py default) is surfaced here in Phase C, when the
-    # incremental watch-sync scheduler job that reads it lands.
+    # NOTE: plex_watch_sync_interval_min is a config.py default read by the watch-sync scheduler job
+    # (scheduler.py); it is intentionally NOT a runtime admin knob, so it stays out of SPECS.
 )
 
 _BY_KEY: dict[str, ConfigSpec] = {s.key: s for s in SPECS}