feat: Implement SQLAlchemy database layer with comprehensive models

Implemented a complete database layer for persistent storage of anime series,
episodes, download queue, and user sessions using SQLAlchemy ORM.

Features:
- 4 SQLAlchemy models: AnimeSeries, Episode, DownloadQueueItem, UserSession
- Automatic timestamp tracking via TimestampMixin
- Foreign key relationships with cascade deletes
- Async and sync database session support
- FastAPI dependency injection integration
- SQLite optimizations (WAL mode, foreign keys)
- Enum types for status and priority fields

Models:
- AnimeSeries: Series metadata with one-to-many relationships
- Episode: Individual episodes linked to series
- DownloadQueueItem: Queue persistence with progress tracking
- UserSession: JWT session storage with expiry and revocation

Database Management:
- Async engine creation with aiosqlite
- Session factory with proper lifecycle
- Connection pooling configuration
- Automatic table creation on initialization

Testing:
- 19 comprehensive unit tests (all passing)
- In-memory SQLite for test isolation
- Relationship and constraint validation
- Query operation testing

Documentation:
- Comprehensive database section in infrastructure.md
- Database package README with examples
- Implementation summary document
- Usage guides and troubleshooting

Dependencies:
- Added: sqlalchemy>=2.0.35 (Python 3.13 compatible)
- Added: alembic==1.13.0 (for future migrations)
- Added: aiosqlite>=0.19.0 (async SQLite driver)

Files:
- src/server/database/__init__.py (package exports)
- src/server/database/base.py (base classes and mixins)
- src/server/database/models.py (ORM models, ~435 lines)
- src/server/database/connection.py (connection management)
- src/server/database/migrations.py (migration placeholder)
- src/server/database/README.md (package documentation)
- tests/unit/test_database_models.py (19 test cases)
- DATABASE_IMPLEMENTATION_SUMMARY.md (implementation summary)

Closes #9 Database Layer implementation task

src/server/database/__init__.py

@@ -0,0 +1,42 @@
+"""Database package for the Aniworld web application.
+
+This package provides SQLAlchemy models, database connection management,
+and session handling for persistent storage.
+
+Modules:
+    - models: SQLAlchemy ORM models for anime series, episodes, download queue, and sessions
+    - connection: Database engine and session factory configuration
+    - base: Base class for all SQLAlchemy models
+
+Usage:
+    from src.server.database import get_db_session, init_db
+    
+    # Initialize database on application startup
+    init_db()
+    
+    # Use in FastAPI endpoints
+    @app.get("/anime")
+    async def get_anime(db: AsyncSession = Depends(get_db_session)):
+        result = await db.execute(select(AnimeSeries))
+        return result.scalars().all()
+"""
+
+from src.server.database.base import Base
+from src.server.database.connection import close_db, get_db_session, init_db
+from src.server.database.models import (
+    AnimeSeries,
+    DownloadQueueItem,
+    Episode,
+    UserSession,
+)
+
+__all__ = [
+    "Base",
+    "get_db_session",
+    "init_db",
+    "close_db",
+    "AnimeSeries",
+    "Episode",
+    "DownloadQueueItem",
+    "UserSession",
+]