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  superseded reels' mp4+sidecar.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>

.env.example

@@ -160,3 +160,10 @@ SEARCH_RAG_RERANK=0
 # REEL_PREGEN_LIBRARY=all
 # Max agentic tool iterations for pre-gen scripter. Default 8.
 # REEL_PREGEN_MAX_TOOL_ITERS=8
+#
+# On-disk reel cache sweep (runs every 24h, independent of pre-gen). Removes
+# reel MP4s with no ledger row + no live job that are older than the max age —
+# i.e. the on-demand cache, which otherwise grows forever. Set to 0 to disable.
+# REEL_CACHE_SWEEP_ENABLED=1
+# Age (days) before an unreferenced reel MP4 is swept. Default 7.
+# REEL_CACHE_MAX_AGE_DAYS=7