fix: M4 Phase 1 bugs - dev.chunks refs, search_path, uuid column

Bug fixes from M4 report:
- 4 remaining dev.chunks → dev.chunk in SQL queries
- search_path includes public for pgvector extension
- get_chunk_by_chunk_id_and_uuid: uuid → file_uuid
- New endpoint: GET /api/v1/file/:uuid/chunk/:chunk_id

src/worker/processor.rs

@@ -1080,7 +1080,7 @@ impl ProcessorPool {
                 "top_5": scene.top_5,
             });
             let _ = sqlx::query(
-                "UPDATE dev.chunks SET metadata = metadata || $1::jsonb WHERE file_uuid=$2 AND chunk_id=$3"
+                "UPDATE dev.chunk SET metadata = metadata || $1::jsonb WHERE file_uuid=$2 AND chunk_id=$3"
             )
             .bind(&meta)
             .bind(uuid)