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

alembic/versions/0101_clear_failure_state_on_disabled_sources.py

@@ -0,0 +1,66 @@
+"""Clear failure state on sources that are disabled (#4279).
+
+`failing_sources_clause()` now means "enabled AND erroring", so a disabled
+source no longer counts as failing. That fixes what the surfaces REPORT; it
+does not touch what the rows already CARRY, and the rows are the reason the
+operator saw a banner for six days with no way to act on it (lesson #4202 —
+a guard does not undo the value already stored).
+
+## The row this exists for
+
+Ebi77 (source 19): the membership sweep stopped it as `former_patron` at
+02:50 on 2026-09-15 and correctly cleared its failure state. A deep scan was
+armed twenty minutes later — `/backfill` had no `enabled` guard, which this
+release also fixes — and could not complete without access, so the recovery
+sweep stranded it:
+
+    consecutive_failures = 1
+    last_error = "stranded by recovery sweep (no terminal status after time_limit)"
+
+Nothing could clear that. A disabled source is never scheduled, so no
+successful run resets the counter; `SourceService.update` clears failure
+state only on an explicit disable, and the source was already disabled; and
+the card's Retry routes to `/check`, which refuses a disabled source.
+
+## Why every disabled source, not just that one
+
+The clear matches what `SourceService.update` already does when a source is
+disabled through the app — "disable the subs you're not paying for without
+them lingering as failing" — so this brings rows disabled by any OTHER path
+(the membership sweep, a retired platform in 0097) into line with the rows
+disabled by hand. Same shape as 0097: a repair migration reaches the live
+instance on deploy rather than waiting for someone to find the row.
+
+Enabled sources are untouched — a real failure on a live source must keep
+showing.
+
+Revision ID: 0101
+Revises: 0100
+Create Date: 2026-09-21
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0101"
+down_revision: Union[str, None] = "0100"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute(
+        "UPDATE source SET last_error = NULL, error_type = NULL, "
+        "consecutive_failures = 0 "
+        "WHERE NOT enabled "
+        "AND (last_error IS NOT NULL OR error_type IS NOT NULL "
+        "     OR consecutive_failures <> 0)"
+    )
+
+
+def downgrade() -> None:
+    # Irreversible by design: the cleared strings and counts are not recorded
+    # anywhere, and restoring a failure state nobody can act on would only
+    # re-create the banner this removes. Rule #22 owes no story backwards.
+    pass