Reels: bound disk/ledger growth (pre-gen prune + on-demand cache sweep)

Nothing reaped reels before, so the on-disk cache and ledger grew
unbounded — each night's daily reel is a new ~4MB file + ledger row that's
stale within ~26h.

- Pre-gen self-prune: after recording a reel, prune_superseded keeps the
  newest PREGEN_KEEP_PER_SCOPE (2) rows per (span, library) and unlinks the
  superseded reels' mp4+sidecar. Caps the ledger/disk at ~spans×libraries×2.
- On-disk sweeper (spawn_reel_cache_sweeper): every 24h, removes reel mp4s
  with no ledger row and no live job older than REEL_CACHE_MAX_AGE_DAYS (7) —
  bounding the on-demand cache, which has no ledger row and otherwise grows
  forever — plus crashed-render cruft (.mp4.tmp/.concat.txt/orphan sidecars).
  Runs regardless of REEL_PREGEN_ENABLED; disable with REEL_CACHE_SWEEP_ENABLED=0.
- New DAO methods prune_superseded + all_cache_keys (with tests); env knobs
  documented in .env.example.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -269,6 +269,8 @@ fn main() -> std::io::Result<()> {
 
         // Spawn the nightly pre-generation scheduler (Section D).
         reels::spawn_pregen_scheduler(app_state.clone()).await;
+        // Spawn the on-disk reel-cache sweeper (bounds pre-gen + on-demand reels).
+        reels::spawn_reel_cache_sweeper(app_state.clone()).await;
 
         HttpServer::new(move || {
             let user_dao = SqliteUserDao::new();