Add Pytest QA suite for all endpoints + known-gap regressions - #1

Summary

Adds test_python_fastapi.py — a self-contained Pytest suite (40 tests) covering all 12 API endpoints (happy path, negative path, auth-required, wrong-credentials) plus targeted regression tests for the Critical/High QA gaps found earlier. No application code is changed — this PR only adds tests.

Result: 34 passed, 1 skipped, 5 xfailed (suite is green).

Known bugs are asserted at their correct expected behaviour and marked xfail so the suite stays green while documenting each defect. If a bug is later fixed, the test XPASSes (strict), signalling the marker should be removed. Gaps that need an infra change to test are skip placeholders.

Gap-specific tests:

• IDOR — bob cannot modify/delete alice's post (these pass — ownership is checked). bob reading alice's post via GET /{id} and seeing it in GET / are xfail (no ownership check → confidentiality bug).
• update_post 404 — PUT /api/posts/{missing} should return 404; currently returns 200 (xfail strict).
• Invalid UUID — GET /api/posts/not-a-uuid should return 400; currently 500 (xfail strict).
• POST validation — missing fields → 422 (passes); empty strings → 422 is xfail (schema has no min_length).
• Login rate limit — skip placeholder asserting a future 429; needs a limiter (slowapi/Redis) to test.

Test harness notes:

# isolated DB derived from settings, created if absent
TEST_DATABASE_URL = <base>/<POSTGRES_DB>_test
# per-test TRUNCATE ... RESTART IDENTITY CASCADE for a clean slate
# get_db overridden via app.dependency_overrides
# SMTP patched out (register): app.routers.auth.Email -> fake no-op
# auth via Authorization: Bearer <access_token> from /login
# TestClient(app, raise_server_exceptions=False) so a buggy 500 is
#   returned as a response instead of bubbling up

Run:

pytest test_python_fastapi.py -v   # requires reachable Postgres (see .env / docker-compose.yml)

Link to Devin session: https://app.devin.ai/sessions/00a96a01db324b9583129cd1fd0e9ac4
Requested by: @tejasri757