Feat: Enforce PostgreSQL for integration tests; add Docker test stack

- conftest.py: pytest_configure guard rejects non-postgresql+asyncpg:// URLs
  before any test collects (per constitution §2.5/§5.2 v1.3.0)
- docker-compose.test.yml: isolated postgres-test (5433) + minio-test (9002)
  + api-test runner; one command runs the full suite against real PostgreSQL
- Makefile: test-unit and test-integration targets
- .env.test.example: documents variables needed to run tests outside Docker
- Fix pre-existing test bug: integration tests using client fixture (NoOpAuthProvider)
  for write operations (upload/delete/patch) now use authed_client with Bearer
  token — these were never caught because tests never ran against a live stack

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

specs/008-postgres-integration-tests/quickstart.md

@@ -0,0 +1,38 @@
+# Quickstart: Integration Test Infrastructure
+
+## Run the full integration test suite (Docker, recommended)
+
+```bash
+docker compose -f docker-compose.test.yml run --rm api-test
+```
+
+Test services start automatically. The command exits with pytest's return code.
+
+## Run unit tests only (no Docker required)
+
+```bash
+make test-unit
+# or directly:
+cd api && python -m pytest tests/unit/ -v
+```
+
+## Run integration tests from the host (test services must be running)
+
+```bash
+# Start test services
+docker compose -f docker-compose.test.yml up -d postgres-test minio-test minio-init-test
+
+# Copy and source test env vars
+cp .env.test.example .env.test
+export $(cat .env.test | xargs)
+
+# Run tests
+cd api && python -m pytest tests/integration/ -v
+```
+
+## Validate the guard works
+
+```bash
+TEST_DATABASE_URL=sqlite+aiosqlite:///test.db python -m pytest api/tests/integration/
+# Expected: exits immediately with "Integration tests require postgresql+asyncpg://"
+```