tests: extract _build_session_factory to a shared conftest fixture

The same per-test sqlite-engine setup was duplicated across 14 test
files (~30 lines each). Consolidated into a single async fixture
`db_factory` in tests/conftest.py; tests now take db_factory as a
parameter and use `async with db_factory() as session` directly.

No behaviour change — same function-scope, same in-memory schema
created via Base.metadata.create_all, same app.db._engine /
_session_factory rebinding so module-level helpers see the test
engine. Just ~420 lines of boilerplate removed.

tests/conftest.py

@@ -17,3 +17,40 @@ sys.path.insert(0, str(ROOT))
 # Sentinel env so importing app.config doesn't try to read a missing .env.
 os.environ.setdefault("DATABASE_URL", "sqlite+aiosqlite:///:memory:")
 os.environ.setdefault("CASSANDRA_MOCK", "1")
+
+
+import pytest
+
+
+@pytest.fixture
+async def db_factory(tmp_path):
+    """Per-test sqlite engine + async session factory.
+
+    Creates a fresh sqlite database file under ``tmp_path``, applies
+    ``Base.metadata.create_all``, and rebinds ``app.db._engine`` /
+    ``app.db._session_factory`` so module-level helpers (which look
+    these up at call time) see the test engine.
+
+    Yields the ``async_sessionmaker``. Tests use it like:
+
+        async def test_foo(db_factory):
+            async with db_factory() as session:
+                ...
+    """
+    from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+
+    from app import db as db_mod
+    from app.db import Base
+    import app.models  # noqa: F401 — registers models on Base.metadata
+
+    engine = create_async_engine(f"sqlite+aiosqlite:///{tmp_path}/test.db")
+    factory = async_sessionmaker(engine, expire_on_commit=False)
+    db_mod._engine = engine
+    db_mod._session_factory = factory
+
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+
+    yield factory
+
+    await engine.dispose()