feat(api): folder quota helpers with atomic token usage

Implements check_folder_quota and add_token_usage in app/billing/quota.py
with dialect-aware upsert (pg_insert on PostgreSQL, read-then-write on SQLite).
Adds test_user_free/test_user_power fixtures and db alias to conftest.py.
6 new tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -17,6 +17,8 @@ from jose import jwt
 from sqlalchemy import StaticPool, event
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 
+from sqlalchemy import select
+
 from app.config.settings import settings
 from app.db import Base, get_session
 from app.main import app
@@ -134,6 +136,32 @@ def auth_header(tier: str = "power", user_id: str | None = None) -> dict[str, st
     return {"Authorization": f"Bearer {make_jwt(tier, user_id)}"}
 
 
+# ── Convenience aliases and per-tier user fixtures ────────────────────
+
+@pytest_asyncio.fixture
+async def db(db_session: AsyncSession) -> AsyncSession:
+    """Alias for db_session — used by folder quota tests."""
+    return db_session
+
+
+@pytest_asyncio.fixture
+async def test_user_free(db_session: AsyncSession):
+    """Return the seeded free-tier User row."""
+    result = await db_session.execute(
+        select(User).where(User.id == TEST_USER_IDS["free"])
+    )
+    return result.scalar_one()
+
+
+@pytest_asyncio.fixture
+async def test_user_power(db_session: AsyncSession):
+    """Return the seeded power-tier User row."""
+    result = await db_session.execute(
+        select(User).where(User.id == TEST_USER_IDS["power"])
+    )
+    return result.scalar_one()
+
+
 # ── CLI options ───────────────────────────────────────────────────────
 
 def pytest_addoption(parser):