Pivot per audit: add nginx vod_response_cache, drop premature DB indexes

- Add vod_response_cache (64m) to nginx-vod-module config, directly
  targeting the maintainer-identified bottleneck of manifest/playlist
  generation on historical playback cold opens
- Remove migration 036 (composite recording indexes) — existing indexes
  are already sufficient; schema expansion deferred until profiling
  justifies it

This aligns with the V2 limited-scope patch plan:
  1. nginx VOD response cache (this commit)
  2. vod_ts overlap predicate rewrite (prior commit)
  3. in-process historical mapping memoization (prior commit)
  4. sibling path cleanup in recording_clip (prior commit)

https://claude.ai/code/session_01XeVxvSk9ywyPBR288ZZXWE

docker/main/rootfs/usr/local/nginx/conf/nginx.conf

@@ -94,6 +94,7 @@ http {
         # vod caches
         vod_metadata_cache metadata_cache 512m;
         vod_mapping_cache mapping_cache 5m 10m;
+        vod_response_cache response_cache 64m;
 
         # gzip manifests
         gzip on;

migrations/036_add_recordings_vod_indexes.py

@@ -1,30 +0,0 @@
-"""Peewee migrations -- 036_add_recordings_vod_indexes.py.
-
-Add indexes to the recordings table optimized for the VOD overlap query:
-  WHERE camera = ? AND end_time > ? AND start_time < ?
-  ORDER BY start_time ASC
-
-The composite index (camera, start_time, end_time) covers the full predicate
-and ordering in a single B-tree walk. The (camera, end_time) index gives the
-planner an alternative access path for the end_time > ? filter.
-"""
-
-import peewee as pw
-
-SQL = pw.SQL
-
-
-def migrate(migrator, database, fake=False, **kwargs):
-    migrator.sql(
-        'CREATE INDEX IF NOT EXISTS "idx_recordings_camera_start_end" '
-        'ON "recordings" ("camera", "start_time", "end_time")'
-    )
-    migrator.sql(
-        'CREATE INDEX IF NOT EXISTS "idx_recordings_camera_end" '
-        'ON "recordings" ("camera", "end_time")'
-    )
-
-
-def rollback(migrator, database, fake=False, **kwargs):
-    migrator.sql('DROP INDEX IF EXISTS "idx_recordings_camera_start_end"')
-    migrator.sql('DROP INDEX IF EXISTS "idx_recordings_camera_end"')