fix: a stopped source is not a failing one, and cannot be deep-scanned (4279)

Ebi77 sat in the "1 source is failing" banner for six days with no action
available, reading `stranded by recovery sweep (no terminal status after
time_limit)`. Four things lined up:

1. The membership sweep did its job — saw `former_patron`, disabled the
   source, cleared its failure state. Clean at 02:50.
2. Twenty minutes later a deep scan was armed on it. `/backfill` had a
   credential pre-flight but NO `enabled` guard, while `/check` has carried
   one all along. The two trigger endpoints disagreed, and the ungated one is
   the one that arms the long walk.
3. Without a membership the walk cannot finish, never reaches a terminal
   status, and the recovery sweep strands it with consecutive_failures = 1.
4. Nothing could clear that. A disabled source is never scheduled, so no
   successful run resets the count; `SourceService.update` clears only on an
   explicit disable and it was already disabled; and the banner's Retry routes
   to `/check`, which refuses a disabled source. The card offered a button
   structurally incapable of acting on the only source it was showing.

`failing_sources_clause()` now means "enabled AND erroring". That also settles
a disagreement its two callers already had: the scheduler's count paired it
with `enabled.is_(True)` and `SourceService.list(failing=True)` did not, so
one counted Ebi77 and the other did not — exactly the drift the note above
that function warns about, which is why the test belongs IN the predicate
rather than beside it. The scheduler's now-duplicate clause is dropped so one
place decides.

`/backfill` gains the guard for start/recover/recapture. `stop` stays open on
a disabled source, or arming becomes a one-way door.

Migration 0101 clears failure state on sources that are already disabled — the
predicate fixes what the surfaces report, not what the rows carry, and the
rows are why the operator had no way out (lesson #4202). It matches what
`update` already does on an explicit disable, so rows disabled by any other
path come into line. Enabled sources are untouched: a real failure on a live
source must keep showing, which the second new test pins.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/api/sources.py

@@ -201,6 +201,16 @@ async def set_backfill(source_id: int):
             rec = await SourceService(session).get(source_id)
             if rec is None:
                 return _bad("not_found", status=404)
+            # A disabled source must not be armable for a deep walk — the same
+            # rule /check has carried all along (see `source_disabled` below).
+            # Arming one anyway is how #4279 happened: the membership sweep had
+            # stopped Ebi77 as `former_patron`, a deep scan was armed twenty
+            # minutes later, the walk could not complete without access, and
+            # the recovery sweep stranded it with a failure count no surface
+            # could clear — a disabled source is never scheduled again, and
+            # Retry routes to /check, which refuses it.
+            if not rec.enabled:
+                return _bad("source_disabled", detail="enable the source first")
             native = uses_native_ingester(rec.platform)
             if native:
                 cred = CredentialService(session, _get_crypto())