fix(subscribestar): route .art creators to .adult; clear source failure on disable

Two pre-merge fixes:

1. SubscribeStar .art age wall: the 18+ cookie doesn't clear the age gate on
   the .art domain (keeps 302'ing to /age_confirmation_warning even with the
   cookie — Elasid #54116), but the same creator is reachable on .adult where
   the cookie works. _normalize_ss_host rewrites subscribestar.art →
   subscribestar.adult at request time (stored Source.url untouched), logged so
   it's visible in walk logs. .com/.adult pass through.

2. Disabling a source now clears its failure state (last_error, error_type,
   consecutive_failures) so subs you pause (not paying for) stop lingering as
   'failing'. Only the explicit disable clears — an unrelated edit to an
   already-disabled source leaves state alone.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/source_service.py

@@ -298,6 +298,15 @@ class SourceService:
 
         for key, value in fields.items():
             setattr(source, key, value)
+        # Disabling a source clears its failure state (operator: disable the subs
+        # you're not paying for without them lingering as "failing"). Re-enabling
+        # then starts clean; the next real run re-derives health. Only on the
+        # explicit disable — an unrelated edit to an already-disabled source
+        # leaves its (already-clear) state alone.
+        if fields.get("enabled") is False:
+            source.last_error = None
+            source.error_type = None
+            source.consecutive_failures = 0
         try:
             await self.session.flush()
         except IntegrityError as exc: