feat: add conversation persistence and context management layer

- SQLAlchemy async database layer (SQLite dev, PostgreSQL prod)
- Conversation and Message models with UUID primary keys
- Token counting utilities using litellm
- Context summarization at 80% token threshold
- REST API endpoints for multi-turn conversations
- 19 conversation tests, 6 token tests (176 total passing)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/db/database.py

@@ -0,0 +1,123 @@
+"""
+Async SQLAlchemy database management.
+
+Pattern from core-api: singleton Database class with async session factory.
+"""
+from collections.abc import AsyncGenerator
+from functools import lru_cache
+
+from sqlalchemy.ext.asyncio import (
+    AsyncEngine,
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+
+from src.shared.config import get_settings
+from src.shared.logging import get_logger
+
+logger = get_logger(__name__)
+
+
+class Database:
+    """
+    Async database connection manager.
+
+    Manages SQLAlchemy async engine and session factory.
+    """
+
+    def __init__(self, url: str):
+        """
+        Initialize database with connection URL.
+
+        Args:
+            url: SQLAlchemy async connection URL
+                 e.g., "sqlite+aiosqlite:///./webber.db"
+                 or "postgresql+asyncpg://user:pass@host/db"
+        """
+        self._url = url
+        self._engine: AsyncEngine | None = None
+        self._session_factory: async_sessionmaker[AsyncSession] | None = None
+
+    @property
+    def engine(self) -> AsyncEngine:
+        """Get or create the async engine."""
+        if self._engine is None:
+            self._engine = create_async_engine(
+                self._url,
+                echo=get_settings().debug,
+                pool_pre_ping=True,
+            )
+        return self._engine
+
+    @property
+    def session_factory(self) -> async_sessionmaker[AsyncSession]:
+        """Get or create the session factory."""
+        if self._session_factory is None:
+            self._session_factory = async_sessionmaker(
+                bind=self.engine,
+                class_=AsyncSession,
+                expire_on_commit=False,
+                autoflush=False,
+            )
+        return self._session_factory
+
+    async def create_tables(self) -> None:
+        """Create all tables (for development)."""
+        from src.db.models import Base
+
+        async with self.engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+        logger.info("Database tables created")
+
+    async def close(self) -> None:
+        """Close the database connection."""
+        if self._engine:
+            await self._engine.dispose()
+            self._engine = None
+            self._session_factory = None
+            logger.info("Database connection closed")
+
+
+# Singleton instance
+_database: Database | None = None
+_tables_created: bool = False
+
+
+@lru_cache
+def get_database() -> Database:
+    """Get the database singleton."""
+    global _database
+    if _database is None:
+        settings = get_settings()
+        _database = Database(settings.database_url)
+    return _database
+
+
+async def _ensure_tables() -> None:
+    """Ensure database tables exist (lazy initialization)."""
+    global _tables_created
+    if not _tables_created:
+        database = get_database()
+        await database.create_tables()
+        _tables_created = True
+
+
+async def get_session() -> AsyncGenerator[AsyncSession, None]:
+    """
+    Dependency for getting async database sessions.
+
+    Usage:
+        @router.get("/")
+        async def endpoint(session: AsyncSession = Depends(get_session)):
+            ...
+    """
+    await _ensure_tables()
+    database = get_database()
+    async with database.session_factory() as session:
+        try:
+            yield session
+            await session.commit()
+        except Exception:
+            await session.rollback()
+            raise