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/README.md

@@ -4,7 +4,7 @@ SQLAlchemy-based database layer for the Aniworld web application.
 
 ## Overview
 
-This package provides persistent storage for anime series, episodes, download queue, and user sessions using SQLAlchemy ORM.
+This package provides persistent storage for anime series, episodes, download queue, and user sessions using SQLAlchemy ORM with comprehensive service layer for CRUD operations.
 
 ## Quick Start
 
@@ -198,6 +198,149 @@ The test suite uses an in-memory SQLite database for isolation and speed.
 -   **connection.py**: Engine, session factory, dependency injection
 -   **migrations.py**: Alembic migration placeholder
 -   ****init**.py**: Package exports
+-   **service.py**: Service layer with CRUD operations
+
+## Service Layer
+
+The service layer provides high-level CRUD operations for all models:
+
+### AnimeSeriesService
+
+```python
+from src.server.database import AnimeSeriesService
+
+# Create series
+series = await AnimeSeriesService.create(
+    db,
+    key="my-anime",
+    name="My Anime",
+    site="https://example.com",
+    folder="/path/to/anime"
+)
+
+# Get by ID or key
+series = await AnimeSeriesService.get_by_id(db, series_id)
+series = await AnimeSeriesService.get_by_key(db, "my-anime")
+
+# Get all with pagination
+all_series = await AnimeSeriesService.get_all(db, limit=50, offset=0)
+
+# Update
+updated = await AnimeSeriesService.update(db, series_id, name="Updated Name")
+
+# Delete (cascades to episodes and downloads)
+deleted = await AnimeSeriesService.delete(db, series_id)
+
+# Search
+results = await AnimeSeriesService.search(db, "naruto", limit=10)
+```
+
+### EpisodeService
+
+```python
+from src.server.database import EpisodeService
+
+# Create episode
+episode = await EpisodeService.create(
+    db,
+    series_id=1,
+    season=1,
+    episode_number=5,
+    title="Episode 5"
+)
+
+# Get episodes for series
+episodes = await EpisodeService.get_by_series(db, series_id, season=1)
+
+# Get specific episode
+episode = await EpisodeService.get_by_episode(db, series_id, season=1, episode_number=5)
+
+# Mark as downloaded
+updated = await EpisodeService.mark_downloaded(
+    db,
+    episode_id,
+    file_path="/path/to/file.mp4",
+    file_size=1024000
+)
+```
+
+### DownloadQueueService
+
+```python
+from src.server.database import DownloadQueueService
+from src.server.database.models import DownloadPriority, DownloadStatus
+
+# Add to queue
+item = await DownloadQueueService.create(
+    db,
+    series_id=1,
+    season=1,
+    episode_number=5,
+    priority=DownloadPriority.HIGH
+)
+
+# Get pending downloads (ordered by priority)
+pending = await DownloadQueueService.get_pending(db, limit=10)
+
+# Get active downloads
+active = await DownloadQueueService.get_active(db)
+
+# Update status
+updated = await DownloadQueueService.update_status(
+    db,
+    item_id,
+    DownloadStatus.DOWNLOADING
+)
+
+# Update progress
+updated = await DownloadQueueService.update_progress(
+    db,
+    item_id,
+    progress_percent=50.0,
+    downloaded_bytes=500000,
+    total_bytes=1000000,
+    download_speed=50000.0
+)
+
+# Clear completed
+count = await DownloadQueueService.clear_completed(db)
+
+# Retry failed downloads
+retried = await DownloadQueueService.retry_failed(db, max_retries=3)
+```
+
+### UserSessionService
+
+```python
+from src.server.database import UserSessionService
+from datetime import datetime, timedelta
+
+# Create session
+expires_at = datetime.utcnow() + timedelta(hours=24)
+session = await UserSessionService.create(
+    db,
+    session_id="unique-session-id",
+    token_hash="hashed-jwt-token",
+    expires_at=expires_at,
+    user_id="user123",
+    ip_address="127.0.0.1"
+)
+
+# Get session
+session = await UserSessionService.get_by_session_id(db, "session-id")
+
+# Get active sessions
+active = await UserSessionService.get_active_sessions(db, user_id="user123")
+
+# Update activity
+updated = await UserSessionService.update_activity(db, "session-id")
+
+# Revoke session
+revoked = await UserSessionService.revoke(db, "session-id")
+
+# Cleanup expired sessions
+count = await UserSessionService.cleanup_expired(db)
+```
 
 ## Database Schema