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;