fix: support in-memory SQLite and ensure custom MIME types are initialized

- Add _init_memory_db() path using Base.metadata.create_all + StaticPool
  since Alembic migrations don't work with in-memory SQLite (each
  connection gets its own separate database)
- Call init_mime_types() at module load in metadata_extract so custom
  types like application/safetensors are always registered

Amp-Thread-ID: https://ampcode.com/threads/T-019cbb5f-13d1-7429-8cfd-815625c4d032
Co-authored-by: Amp <amp@ampcode.com>

app/assets/services/metadata_extract.py

@@ -14,6 +14,10 @@ import struct
 from dataclasses import dataclass
 from typing import Any
 
+from utils.mime_types import init_mime_types
+
+init_mime_types()
+
 # Supported safetensors extensions
 SAFETENSORS_EXTENSIONS = frozenset({".safetensors", ".sft"})