feat(database): Implement comprehensive database service layer

Implemented database service layer with CRUD operations for all models:

- AnimeSeriesService: Create, read, update, delete, search anime series
- EpisodeService: Episode management and download tracking
- DownloadQueueService: Priority-based queue with status tracking
- UserSessionService: Session management with JWT support

Features:
- Repository pattern for clean separation of concerns
- Full async/await support for non-blocking operations
- Comprehensive type hints and docstrings
- Transaction management via FastAPI dependency injection
- Priority queue ordering (HIGH > NORMAL > LOW)
- Automatic timestamp management
- Cascade delete support

Testing:
- 22 comprehensive unit tests with 100% pass rate
- In-memory SQLite for isolated testing
- All CRUD operations tested

Documentation:
- Enhanced database README with service examples
- Integration examples in examples.py
- Updated infrastructure.md with service details
- Migration utilities for schema management

Files:
- src/server/database/service.py (968 lines)
- src/server/database/examples.py (467 lines)
- tests/unit/test_database_service.py (22 tests)
- src/server/database/migrations.py (enhanced)
- src/server/database/__init__.py (exports added)

Closes #9 - Database Layer: Create database service

src/server/database/__init__.py

@@ -29,6 +29,12 @@ from src.server.database.models import (
     Episode,
     UserSession,
 )
+from src.server.database.service import (
+    AnimeSeriesService,
+    DownloadQueueService,
+    EpisodeService,
+    UserSessionService,
+)
 
 __all__ = [
     "Base",
@@ -39,4 +45,8 @@ __all__ = [
     "Episode",
     "DownloadQueueItem",
     "UserSession",
+    "AnimeSeriesService",
+    "EpisodeService",
+    "DownloadQueueService",
+    "UserSessionService",
 ]