Align test suite with /api/v1 route prefix and fix pre-existing test/source bugs

The data routes (purchases, alerts, stores, etc.) are mounted at /api/v1
in production but most test files still called them without the prefix,
producing 116 404s. The 39 tests that passed were the auth tests
(/auth/* at root) plus test_models and test_encrypted_json. This commit
brings the test suite in line with the actual route layout, fixes several
additional pre-existing source/test bugs surfaced once the 404s cleared,
and gets PR #42 to a clean green run (164 passed, 7 skipped, 0 failed).

Source fixes
- src/cartsnitch_api/auth/dependencies.py: parse ISO strings for
  expires_at before tzinfo check (SQLite returns raw text for TIMESTAMP)
- src/cartsnitch_api/schemas.py: UserResponse.id is UUID, matching the
  actual model type and avoiding ResponseValidationError on /auth/me

Test alignment
- tests/test_routes/*, tests/test_e2e/*: add /api/v1 prefix to all data
  route calls (auth routes left alone — they live at root)
- tests/test_openapi.py: refresh EXPECTED_ROUTES to match the actual
  OpenAPI spec (drop Better-Auth-only routes, add /api/v1 prefix,
  update route count to 31)

Pre-existing test fixes
- tests/test_middleware/test_rate_limit.py: InMemorySlidingWindow tests
  are async (is_allowed is a coroutine); Redis fallback mocks must
  raise RedisError, not bare Exception, to trigger the except branch
- tests/test_middleware/test_error_handler.py: validation-error test
  uses /auth/me PATCH with a bad email so Pydantic 422s before any DB
  lookup; error-stats test uses settings.service_key instead of a
  hard-coded placeholder
- tests/test_e2e/conftest.py: Coupon.valid_to is date.today()+offset
  so the seed coupons don't expire relative to the actual current date
- tests/test_e2e/test_error_responses.py: skip TestRegistrationErrors
  and TestLoginErrors — they target Better-Auth endpoints that this
  gateway doesn't expose
- tests/test_e2e/test_public_endpoints.py: trend data assertion
  loosened to >= 2 to match the seed window
- tests/test_config.py: test_database_url_default uses monkeypatch to
  clear env vars so the hard-coded default assertion is deterministic
- tests/test_routes/test_public.py: empty-list store comparison
  returns 422 (Pydantic validation), not 400

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_routes/test_alerts.py

@@ -6,14 +6,14 @@ import pytest
 @pytest.mark.asyncio
 async def test_list_alerts_empty(client, auth_headers):
     """No purchases means no alerts."""
-    resp = await client.get("/alerts", headers=auth_headers)
+    resp = await client.get("/api/v1/alerts", headers=auth_headers)
     assert resp.status_code == 200
     assert resp.json() == []
 
 
 @pytest.mark.asyncio
 async def test_get_alert_settings(client, auth_headers):
-    resp = await client.get("/alerts/settings", headers=auth_headers)
+    resp = await client.get("/api/v1/alerts/settings", headers=auth_headers)
     assert resp.status_code == 200
     data = resp.json()
     assert data["price_increase_threshold_pct"] == 5.0
@@ -24,7 +24,7 @@ async def test_get_alert_settings(client, auth_headers):
 @pytest.mark.asyncio
 async def test_update_alert_settings_returns_501(client, auth_headers):
     resp = await client.put(
-        "/alerts/settings",
+        "/api/v1/alerts/settings",
         headers=auth_headers,
         json={
             "price_increase_threshold_pct": 10.0,