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/tests/test_blueprint_api.py

@@ -0,0 +1,176 @@
+"""
+Integration tests for the blueprint CRUD REST endpoints.
+
+Overrides the database dependency to use an in-memory SQLite database.
+"""
+
+import sys
+import os
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+import pytest
+import pytest_asyncio
+from httpx import AsyncClient, ASGITransport
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from models.blueprint import Base
+from database import get_session
+from main import app
+
+
+# ---------------------------------------------------------------------------
+# Test database setup
+# ---------------------------------------------------------------------------
+
+TEST_DATABASE_URL = "sqlite+aiosqlite:///:memory:"
+test_engine = create_async_engine(TEST_DATABASE_URL, echo=False)
+TestSessionLocal = async_sessionmaker(test_engine, class_=AsyncSession, expire_on_commit=False)
+
+
+async def override_get_session():
+    async with TestSessionLocal() as session:
+        yield session
+
+
+app.dependency_overrides[get_session] = override_get_session
+
+
+@pytest_asyncio.fixture(autouse=True)
+async def setup_db():
+    """Create and tear down tables for each test."""
+    async with test_engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+    yield
+    async with test_engine.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)
+
+
+@pytest_asyncio.fixture
+async def client():
+    transport = ASGITransport(app=app)
+    async with AsyncClient(transport=transport, base_url="http://test") as c:
+        yield c
+
+
+# ---------------------------------------------------------------------------
+# Sample payload
+# ---------------------------------------------------------------------------
+
+SAMPLE_BLUEPRINT = {
+    "version": 1,
+    "name": "Test Council",
+    "nodes": [
+        {
+            "id": "node-1",
+            "label": "Master",
+            "systemPrompt": "You are the master writer.",
+            "model": "claude-3-5-sonnet",
+            "tools": {"webSearch": False, "pdfReader": False},
+            "position": {"x": 0, "y": 0},
+        },
+        {
+            "id": "node-2",
+            "label": "Critic",
+            "systemPrompt": "You evaluate drafts.",
+            "model": "claude-3-5-sonnet",
+            "tools": {"webSearch": False, "pdfReader": False},
+            "position": {"x": 300, "y": 0},
+        },
+    ],
+    "edges": [
+        {"id": "edge-1", "source": "node-1", "target": "node-2", "type": "linear"},
+    ],
+}
+
+
+# ---------------------------------------------------------------------------
+# Tests
+# ---------------------------------------------------------------------------
+
+class TestBlueprintEndpoints:
+    @pytest.mark.asyncio
+    async def test_create_blueprint(self, client):
+        response = await client.post("/api/councils/", json=SAMPLE_BLUEPRINT)
+        assert response.status_code == 201
+        data = response.json()
+        assert data["name"] == "Test Council"
+        assert data["version"] == 1
+        assert len(data["nodes"]) == 2
+        assert "id" in data
+
+    @pytest.mark.asyncio
+    async def test_list_blueprints(self, client):
+        await client.post("/api/councils/", json=SAMPLE_BLUEPRINT)
+        await client.post(
+            "/api/councils/",
+            json={**SAMPLE_BLUEPRINT, "name": "Second Council"},
+        )
+
+        response = await client.get("/api/councils/")
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data) == 2
+
+    @pytest.mark.asyncio
+    async def test_get_blueprint(self, client):
+        create_resp = await client.post("/api/councils/", json=SAMPLE_BLUEPRINT)
+        bp_id = create_resp.json()["id"]
+
+        response = await client.get(f"/api/councils/{bp_id}")
+        assert response.status_code == 200
+        assert response.json()["name"] == "Test Council"
+
+    @pytest.mark.asyncio
+    async def test_get_nonexistent_returns_404(self, client):
+        response = await client.get("/api/councils/nonexistent-id")
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_update_blueprint(self, client):
+        create_resp = await client.post("/api/councils/", json=SAMPLE_BLUEPRINT)
+        bp_id = create_resp.json()["id"]
+
+        update_resp = await client.put(
+            f"/api/councils/{bp_id}",
+            json={"name": "Renamed Council"},
+        )
+        assert update_resp.status_code == 200
+        assert update_resp.json()["name"] == "Renamed Council"
+
+    @pytest.mark.asyncio
+    async def test_update_nonexistent_returns_404(self, client):
+        response = await client.put(
+            "/api/councils/ghost-id",
+            json={"name": "Ghost"},
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_delete_blueprint(self, client):
+        create_resp = await client.post("/api/councils/", json=SAMPLE_BLUEPRINT)
+        bp_id = create_resp.json()["id"]
+
+        delete_resp = await client.delete(f"/api/councils/{bp_id}")
+        assert delete_resp.status_code == 204
+
+        get_resp = await client.get(f"/api/councils/{bp_id}")
+        assert get_resp.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_delete_nonexistent_returns_404(self, client):
+        response = await client.delete("/api/councils/ghost-id")
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_create_rejects_missing_name(self, client):
+        payload = {**SAMPLE_BLUEPRINT}
+        del payload["name"]
+        response = await client.post("/api/councils/", json=payload)
+        assert response.status_code == 422
+
+    @pytest.mark.asyncio
+    async def test_create_rejects_empty_name(self, client):
+        payload = {**SAMPLE_BLUEPRINT, "name": ""}
+        response = await client.post("/api/councils/", json=payload)
+        assert response.status_code == 422