feat: ingest HTTP 201/409 for event_id idempotency

- Validate UUID format; log created/duplicate/rejected

- Tests for 201, 409, 422; update agent-integration and work-plan

- Docs: neutral IDE wording (no product-specific editor names)

backend/tests/conftest.py

@@ -0,0 +1,82 @@
+"""SQLite in-memory fixtures for API tests."""
+
+import os
+
+# Must be set before app.database imports create_engine
+os.environ.setdefault("DATABASE_URL", "sqlite:///:memory:")
+os.environ.setdefault("SAC_BOOTSTRAP_API_KEY", "sac_test_key_for_pytest_only")
+
+import pytest
+from fastapi.testclient import TestClient
+from sqlalchemy import JSON, create_engine, event
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import sessionmaker
+from sqlalchemy.pool import StaticPool
+
+from app.auth.api_key import hash_api_key
+from app.database import Base, get_db
+from app.main import app as fastapi_app
+import app.models  # noqa: F401 — register all tables on Base.metadata
+from app.models import ApiKey
+
+TEST_API_KEY = os.environ["SAC_BOOTSTRAP_API_KEY"]
+
+
+@event.listens_for(Base.metadata, "before_create")
+def _sqlite_jsonb_as_json(metadata, connection, **_kwargs) -> None:
+    if connection.dialect.name != "sqlite":
+        return
+    for table in metadata.tables.values():
+        for column in table.columns:
+            if isinstance(column.type, JSONB):
+                column.type = JSON()
+
+
+@pytest.fixture
+def db_engine():
+    engine = create_engine(
+        "sqlite:///:memory:",
+        connect_args={"check_same_thread": False},
+        poolclass=StaticPool,
+    )
+    Base.metadata.create_all(engine)
+    yield engine
+    engine.dispose()
+
+
+@pytest.fixture
+def db_session(db_engine):
+    Session = sessionmaker(bind=db_engine, autocommit=False, autoflush=False)
+    session = Session()
+    session.add(
+        ApiKey(
+            name="test",
+            key_prefix=TEST_API_KEY[:12],
+            key_hash=hash_api_key(TEST_API_KEY),
+            is_active=True,
+        )
+    )
+    session.commit()
+    yield session
+    session.close()
+
+
+@pytest.fixture
+def client(db_session, monkeypatch):
+    monkeypatch.setattr("app.main.bootstrap_api_key", lambda: None)
+
+    def override_get_db():
+        try:
+            yield db_session
+        finally:
+            pass
+
+    fastapi_app.dependency_overrides[get_db] = override_get_db
+    with TestClient(fastapi_app) as c:
+        yield c
+    fastapi_app.dependency_overrides.clear()
+
+
+@pytest.fixture
+def auth_headers():
+    return {"Authorization": f"Bearer {TEST_API_KEY}"}