feat(patreon): drift detection + error categorization — build step 4 (plan #697)

Typed, loud failure mapping for the native Patreon ingester so a changed
API shape or expired auth never silently zero-downloads as "success".

- New ErrorType.API_DRIFT (free varchar error_type col → no migration):
  distinct from auth so the operator knows the fix is updating the
  ingester, not rotating cookies.
- patreon_client: PatreonAPIError carries status_code; new PatreonAuthError
  for 401/403 + HTML-login/non-JSON bodies (reclassified from drift —
  expired-session is auth, actionable as "rotate cookies").
- patreon_ingester._failure_result maps: PatreonAuthError→AUTH_ERROR,
  PatreonDriftError→API_DRIFT ("Patreon API changed — ingester needs
  update"), HTTP 429→RATE_LIMITED, 404→NOT_FOUND, other HTTP→HTTP_ERROR,
  transport→NETWORK_ERROR. (429 thus drives the platform cooldown.)
- FailingSourcesCard: api_drift chip (red) + hint.

Contract test (new test_patreon_contract.py): the recorded /api/posts
fixture must parse end-to-end (no drift, 5 media across 4 posts) AND the
request params must still carry every field the parser depends on
(file_name, image_urls/download_url, images/attachments_media/media
includes, content/post_file/image post fields) — a trim of either trips a
red build. Plus client HTTP-status classification tests and ingester
error-type mapping tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/FailingSourcesCard.vue

@@ -93,6 +93,7 @@ const ERROR_TYPE_COLOR = {
   unsupported_url: 'error',
   http_error: 'error',
   unknown_error: 'error',
+  api_drift: 'error',
   partial: 'info',
   tier_limited: 'info',
   no_new_content: 'info',
@@ -108,6 +109,7 @@ const ERROR_TYPE_HINT = {
   http_error: 'Generic HTTP error — see Logs.',
   unsupported_url: 'gallery-dl does not support this URL pattern.',
   unknown_error: 'Could not classify — see Logs.',
+  api_drift: 'Patreon changed its API shape — the native ingester needs a code update.',
 }
 function errorTypeColor(t) { return ERROR_TYPE_COLOR[t] || 'error' }
 function errorTypeHint(t) { return ERROR_TYPE_HINT[t] || '' }