feat(01-01): add Pydantic Settings class to config.py and update requirements.txt

- Add Settings(BaseSettings) class reading DATABASE_URL, DATABASE_MIGRATE_URL,
  MINIO_ENDPOINT, MINIO_ACCESS_KEY, MINIO_SECRET_KEY, MINIO_BUCKET, REDIS_URL,
  SECRET_KEY from environment via SettingsConfigDict (pydantic-settings v2 API)
- Instantiate settings = Settings() at module level for all callers
- Preserve legacy DATA_DIR, UPLOADS_DIR, METADATA_DIR, TOPICS_FILE, SETTINGS_FILE,
  DEFAULT_SYSTEM_PROMPT, DEFAULT_SETTINGS, ensure_data_dirs() for Wave 4 compat
- Remove filelock>=3.14 (replaced by PostgreSQL transactions per STORE-07)
- Add sqlalchemy[asyncio]>=2.0.49, psycopg[binary]>=3.3.4, alembic>=1.18.4,
  minio>=7.2.20, celery[redis]>=5.6.3, redis>=7.4.0, aiosqlite>=0.20.0
- Bump pytest-asyncio to >=1.3.0 for asyncio_mode auto support

backend/requirements.txt

@@ -8,8 +8,14 @@ PyMuPDF>=1.24
 python-docx>=1.1
 pytesseract>=0.3
 Pillow>=10.3
-filelock>=3.14
 aiofiles>=23.2
 httpx>=0.27
 pytest>=8.2
-pytest-asyncio>=0.23
+pytest-asyncio>=1.3.0
+sqlalchemy[asyncio]>=2.0.49
+psycopg[binary]>=3.3.4
+alembic>=1.18.4
+minio>=7.2.20
+celery[redis]>=5.6.3
+redis>=7.4.0
+aiosqlite>=0.20.0