perf(db): each feed sort rides an index, and an ICU upgrade rebuilds the title ones

The views, duration and title sorts had no index: every page read the whole videos heap and kept a
top-N (prod 350 ms a page, 550-780 for title). Migration 0096 adds one per direction and, for title,
per reader collation, in the exact ORDER BY feedquery builds. Localdev prod copy: plain sorts
180-310 ms -> ~1 ms, page 20 3-5 ms, narrow filters unchanged. view_count/duration are only written
by the enrich, which already rewrites indexed columns, so no HOT update is lost.

The title indexes are the first on an ICU collation, and postgres:16-alpine floats: collation.heal_drift
rebuilds them (CONCURRENTLY) at startup when Postgres reports the ICU version moved.
test_feed_sort_indexes holds every videos sort to an index and checks the rebuild.

backend/app/main.py

@@ -24,11 +24,12 @@ from sqlalchemy.exc import DataError
 from starlette.middleware.sessions import SessionMiddleware
 
 from app import bodylimit
+from app import collation
 from app import logsink
 from app import state
 from app import sysconfig
 from app.config import settings
-from app.db import SessionLocal
+from app.db import SessionLocal, engine
 from app.plex import stream as plex_stream
 from app.plex import subtitles as plex_subtitles
 from app.routes import (
@@ -97,6 +98,9 @@ async def lifespan(app: FastAPI):
     # owns, so racing it against early playback is safe.
     sweep = asyncio.create_task(_sweep_hls_orphans())
     reaper = asyncio.create_task(_reap_hls_sessions())
+    # Fire and forget as well: a rebuild after an ICU upgrade takes seconds, and the title sorts serve
+    # meanwhile (CONCURRENTLY) in the old index's order, as they did before the restart.
+    heal = asyncio.create_task(_heal_collation_drift())
     start_scheduler()
     try:
         yield
@@ -106,7 +110,7 @@ async def lifespan(app: FastAPI):
         # pending ("Task was destroyed but it is pending!" — noise that reads like a bug during
         # triage). Cancelling does NOT interrupt the rmtree thread itself; this only makes our own
         # bookkeeping honest.
-        for task in (sweep, reaper):
+        for task in (sweep, reaper, heal):
             task.cancel()
             with suppress(asyncio.CancelledError):
                 await task
@@ -139,6 +143,18 @@ async def _reap_hls_sessions() -> None:
             log.exception("HLS session reap failed")
 
 
+async def _heal_collation_drift() -> None:
+    """Background startup check of the ICU collation the title-sort indexes are built in (see
+    `collation.heal_drift`). A failure is logged, never fatal: the sorts still answer, only possibly in
+    the old order."""
+    try:
+        await asyncio.to_thread(collation.heal_drift, engine)
+    except asyncio.CancelledError:
+        raise
+    except Exception:
+        log.exception("ICU collation drift check failed")
+
+
 async def _sweep_hls_orphans() -> None:
     """Background half of the startup HLS sweep (see lifespan). Owns its own errors: a failure here
     must never take the app down, and a cancelled shutdown is not one."""