refactor(discover): one shelf query per kind, one RAIL_LIMIT, a true rollup comment

The comfort shelf resolved each pinned channel with three queries (the
row, its unwatched count, its newest video) and each playlist with two.
channels_summary gets the count and the newest row per channel as window
functions over one filtered set, playlists_summary reads every pinned list
in one query, and the rows come in one IN() each — so the shelf's cost no
longer grows with its length. A test counts the statements.

RAIL_LIMIT had two definitions held in step by a comment; the Plex Discover
route now imports the domain's. And rollup's "continue the latest-started
episode" was false: it takes the last in-progress one in broadcast order,
which is also what Home's next_episode picks.

backend/app/plex/rollup.py

@@ -43,7 +43,9 @@ def rollup(cards: list[dict]) -> dict:
     ]
     resume = None
     if inprog:
-        resume = inprog[-1]  # continue the latest-started episode
+        # The LAST in-progress episode in broadcast order — no timestamp is consulted. Home's
+        # `discover.queries.next_episode` picks the same one, so both Resume buttons agree.
+        resume = inprog[-1]
     else:
         resume = next((c for c in cards if c.get("status") != "watched"), None)
     return {