fix: native FLAC handling and extension API optimizations

Native FLAC handling:
- Properly detect and publish native FLAC payloads inside MP4 containers
- Rename to .flac extension and embed metadata instead of skipping
- Fix all code paths: SAF, non-SAF, and native worker finalizer

Extension API optimizations:
- Enable response compression for API/search calls (faster metadata loads)
- Keep downloads uncompressed for accurate progress/streaming
- Add separate extensionAPITransport with compression enabled

Platform bridge caching:
- Cache handleURLWithExtension results (5 min TTL)
- Cache customSearchWithExtension results (2 min TTL)
- Prevent duplicate in-flight requests for same URL/query

Dependency cleanup:
- Remove unused sqflite_common_ffi and sqlite3 packages

pubspec.lock

@@ -1170,14 +1170,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.5.6+1"
-  sqflite_common_ffi:
-    dependency: "direct main"
-    description:
-      name: sqflite_common_ffi
-      sha256: cd0c7f7de39a08f2d54ef144d9058c46eca8461879aaa648025643455c1e5a20
-      url: "https://pub.dev"
-    source: hosted
-    version: "2.4.0+3"
   sqflite_darwin:
     dependency: transitive
     description:
@@ -1194,14 +1186,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.4.0"
-  sqlite3:
-    dependency: transitive
-    description:
-      name: sqlite3
-      sha256: caa693ad15a587a2b4fde093b728131a1827903872171089dedb16f7665d3a91
-      url: "https://pub.dev"
-    source: hosted
-    version: "3.2.0"
   stack_trace:
     dependency: transitive
     description: