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/services/blueprint_service.py

@@ -0,0 +1,95 @@
+"""
+Blueprint Service — CRUD operations for council blueprints.
+
+Handles persistence of blueprints to PostgreSQL via SQLAlchemy async sessions.
+"""
+
+from datetime import datetime, timezone
+from typing import List, Optional
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from models.blueprint import Blueprint
+
+
+async def create_blueprint(
+    session: AsyncSession,
+    name: str,
+    nodes: list,
+    edges: list,
+    blueprint_id: Optional[str] = None,
+    version: int = 1,
+) -> Blueprint:
+    """Create and persist a new blueprint."""
+    bp = Blueprint(
+        name=name,
+        version=version,
+        nodes=nodes,
+        edges=edges,
+    )
+    if blueprint_id:
+        bp.id = blueprint_id
+
+    session.add(bp)
+    await session.commit()
+    await session.refresh(bp)
+    return bp
+
+
+async def get_blueprint(
+    session: AsyncSession,
+    blueprint_id: str,
+) -> Optional[Blueprint]:
+    """Retrieve a blueprint by ID."""
+    result = await session.execute(
+        select(Blueprint).where(Blueprint.id == blueprint_id)
+    )
+    return result.scalar_one_or_none()
+
+
+async def list_blueprints(session: AsyncSession) -> List[Blueprint]:
+    """Retrieve all blueprints, ordered by most recently updated."""
+    result = await session.execute(
+        select(Blueprint).order_by(Blueprint.updated_at.desc())
+    )
+    return list(result.scalars().all())
+
+
+async def update_blueprint(
+    session: AsyncSession,
+    blueprint_id: str,
+    name: Optional[str] = None,
+    nodes: Optional[list] = None,
+    edges: Optional[list] = None,
+) -> Optional[Blueprint]:
+    """Update an existing blueprint. Returns None if not found."""
+    bp = await get_blueprint(session, blueprint_id)
+    if bp is None:
+        return None
+
+    if name is not None:
+        bp.name = name
+    if nodes is not None:
+        bp.nodes = nodes
+    if edges is not None:
+        bp.edges = edges
+    bp.updated_at = datetime.now(timezone.utc)
+
+    await session.commit()
+    await session.refresh(bp)
+    return bp
+
+
+async def delete_blueprint(
+    session: AsyncSession,
+    blueprint_id: str,
+) -> bool:
+    """Delete a blueprint by ID. Returns True if deleted, False if not found."""
+    bp = await get_blueprint(session, blueprint_id)
+    if bp is None:
+        return False
+
+    await session.delete(bp)
+    await session.commit()
+    return True