fix: delete_video - add file existence check + fix pre_chunks UUID cast

- unregister: check file exists before delete, return 200 with success:false if not found
- delete_video: cast pre_chunks.file_uuid parameter as UUID (::uuid)
- Added Phase 2 test script (10/10 endpoints passed)

src/core/db/postgres_db.rs

@@ -1340,7 +1340,7 @@ impl PostgresDb {
         .await?;
 
         let pre_chunks = schema::table_name("pre_chunks");
-        sqlx::query(&format!("DELETE FROM {} WHERE file_uuid = $1", pre_chunks))
+        sqlx::query(&format!("DELETE FROM {} WHERE file_uuid = $1::uuid", pre_chunks))
             .bind(uuid)
             .execute(&self.pool)
             .await?;