feat(config): add TZ environment variable for container timezone

Adds TZ env var to docker-compose.yml app service (defaults to UTC)
and documents it in .env.example. This ensures the Go runtime's
time.Local is set correctly inside the container for any server-side
time operations that don't use an explicit timezone.

docker-compose.yml

@@ -57,6 +57,9 @@ services:
       BOOKHOARD_CONVERSION_CACHE_DIR: ${BOOKHOARD_CONVERSION_CACHE_DIR:-/app/cache/kepub}
       BOOKHOARD_CONVERSION_TOOL: ${BOOKHOARD_CONVERSION_TOOL:-/usr/bin/kepubify}
       BOOKHOARD_CONVERSION_CACHE_TTL: ${BOOKHOARD_CONVERSION_CACHE_TTL:-24h}
+
+      # System timezone (fallback for server-side time operations)
+      TZ: ${TZ:-UTC}
     ports:
       - "8765:8765"
     depends_on: