Implement Phase 3: dynamic graph builder, blueprint persistence, and CRUD API

- Add dynamic_graph_builder.py that constructs LangGraph graphs at runtime
  from frontend CouncilBlueprint JSON (no more hardcoded graphs in production)
- Add PostgreSQL persistence via SQLAlchemy async with Blueprint model
- Add blueprint CRUD endpoints (POST/GET/PUT/DELETE /api/councils/)
- Add POST /api/councils/{id}/run to execute blueprints dynamically
- Add Alembic migration infrastructure with initial blueprints table
- Add database.py with async engine and SQLite fallback for dev/test
- Fix missing typing-extensions and add aiosqlite dependency
- Add 42 new tests (80/80 total passing) covering dynamic graph building,
  blueprint service CRUD, and API integration

https://claude.ai/code/session_014yZUxrPsgZbvkebXbCXR4U

backend/database.py

@@ -0,0 +1,48 @@
+"""
+Database connection management for CouncilOS.
+
+Provides an async SQLAlchemy engine and session factory backed by PostgreSQL.
+Falls back to SQLite for development/testing if DATABASE_URL is not set.
+"""
+
+import os
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+DATABASE_URL = os.environ.get(
+    "DATABASE_URL",
+    "sqlite+aiosqlite:///./councilOS_dev.db",
+)
+
+# For SQLite async support, replace the driver if needed
+if DATABASE_URL.startswith("sqlite"):
+    connect_args = {"check_same_thread": False}
+else:
+    connect_args = {}
+
+engine = create_async_engine(
+    DATABASE_URL,
+    echo=False,
+    connect_args=connect_args,
+)
+
+async_session = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
+
+
+async def get_session() -> AsyncSession:
+    """Dependency that yields an async database session."""
+    async with async_session() as session:
+        yield session
+
+
+async def init_db() -> None:
+    """Create all tables. Used at application startup."""
+    from models.blueprint import Base
+
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+
+
+async def close_db() -> None:
+    """Dispose of the engine connection pool."""
+    await engine.dispose()