feat(01-02): add async db_session and async_client fixtures to conftest.py

- Add @pytest_asyncio.fixture db_session: in-memory SQLite via aiosqlite,
  expire_on_commit=False, skips gracefully (ImportError) before Plan 03
- Add @pytest_asyncio.fixture async_client: httpx.AsyncClient with
  ASGITransport, overrides deps.db.get_db, skips before Plan 03
- Retain all legacy sync fixtures (isolated_data_dir, client, sample_txt,
  sample_pdf) unchanged for backward compatibility through Plan 04

backend/tests/conftest.py

@@ -1,15 +1,25 @@
 """
 pytest configuration: isolate each test with a temporary data directory.
+
+Async fixtures (db_session, async_client) are added for Phase 1 — sync fixtures remain until Plan 05 cuts over.
 """
+from __future__ import annotations
+
 import os
 import json
 import pytest
+import pytest_asyncio
 import tempfile
 import shutil
 from pathlib import Path
 from fastapi.testclient import TestClient
+from httpx import AsyncClient, ASGITransport
+from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker, AsyncSession
+from sqlalchemy.pool import StaticPool
 
 
+# ── Sync fixtures (legacy — retained until Plan 05 cuts over) ──────────────────
+
 @pytest.fixture(autouse=True)
 def isolated_data_dir(monkeypatch, tmp_path):
     """Each test gets its own clean data directory."""
@@ -68,3 +78,55 @@ def sample_pdf(tmp_path):
     doc.save(str(pdf_path))
     doc.close()
     return pdf_path
+
+
+# ── Async fixtures (Phase 1 additions — Plan 03+ tests use these) ──────────────
+
+@pytest_asyncio.fixture
+async def db_session():
+    """In-memory async SQLite session for unit tests.
+
+    Tries to import db.models.Base (available after Plan 03). If the module
+    does not yet exist the fixture skips the test gracefully so the suite
+    stays green during Wave 1.
+    """
+    engine = create_async_engine(
+        "sqlite+aiosqlite:///:memory:",
+        connect_args={"check_same_thread": False},
+        poolclass=StaticPool,
+    )
+
+    try:
+        from db.models import Base
+        async with engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+    except ImportError:
+        await engine.dispose()
+        pytest.skip("db.models not yet implemented — plan 03")
+
+    AsyncTestSession = async_sessionmaker(engine, expire_on_commit=False)
+    async with AsyncTestSession() as session:
+        yield session
+
+    await engine.dispose()
+
+
+@pytest_asyncio.fixture
+async def async_client(db_session):
+    """Async HTTP test client with DB dependency overridden.
+
+    Tries to import deps.db.get_db (available after Plan 03). If the module
+    does not yet exist the fixture skips the test gracefully.
+    """
+    try:
+        from deps.db import get_db
+        from main import app
+    except ImportError as exc:
+        pytest.skip(f"deps.db.get_db not yet implemented — plan 03: {exc}")
+
+    app.dependency_overrides[get_db] = lambda: db_session
+
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as c:
+        yield c
+
+    app.dependency_overrides.clear()