feat: a source stops pulling once its membership ends, and resumes on resubscribe (3995)

Operator, 2026-09-13: "if I kill a subscription on patreon I would like the pulling to stop on curator as well", with auto-resume chosen. This reverses the 2026-09-11 "report only" decision for lapsed sources.

membership_reconcile.apply_membership_lapses runs in sync_memberships right after each platform's successful sync, so it only ever acts on the roster just written.

It stops a source (enabled=false, with the same failure-state reset as a manual disable, #1285) only when all of these hold:
- the roster is fresh
- the source's matched membership says has_paid_access is False (lapsed, or a free follow)
- the paid-through date has passed, where the platform gives one (Patreon's member.access_expires_at; SubscribeStar gives none, so it stops at once)
- the source is enabled
- the operator hasn't chosen to keep it

It never acts on absence. A source with no matched membership keeps pulling, because a rename or a never-walked source produces the same absence. An unrecognised status is never a lapse either.

It resumes only sources carrying its own `_membership_stopped` marker, once the membership is paid again.

The operator outranks the sweep both ways (SourceService.update):
- turning a stopped source back on marks it `_membership_kept`, so the next sweep leaves it alone until it's paid again
- turning a source off by hand drops the marker, so the sweep never switches it back on

Both are `_`-prefixed app-managed config keys, which operator edits already preserve. No migration.

The roster/fetch line holds. This is a source-level action by the sweep. No download path reads the roster, and the scheduler still selects on `enabled` alone. test_no_fetch_path_can_read_the_roster is unchanged.

UI: SourceRow shows a neutral "Membership ended" chip, with the status and the resume/keep explanation, ahead of the other chips. The sweep's task summary reports stopped/resumed counts.

Tests (tests/test_membership_lapses.py):
- a lapse stops the source with a clean slate and keeps the id cache
- paid-through is honoured
- absence, an unknown status and a stale roster never stop anything
- a resume touches only what the sweep stopped
- a manual on sticks, a manual off drops the marker, and a kept source is released once paid

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

frontend/src/components/subscriptions/SourceRow.vue

@@ -39,8 +39,25 @@
       {{ formatRelative(source.next_check_at, { future: true }) }}
     </td>
     <td>
+      <!-- #3995: the membership sweep stopped this source because paid access
+           ended. First in the chain — it explains why an otherwise healthy
+           source is off. Neutral, never error: nothing is broken. -->
       <v-chip
-        v-if="(source.consecutive_failures || 0) > 0"
+        v-if="membershipStopped"
+        size="x-small" variant="tonal" label
+        prepend-icon="mdi-account-cancel-outline"
+      >Membership ended
+        <v-tooltip activator="parent" location="top" max-width="420">
+          <span>
+            Curator stopped checking this source because your membership
+            {{ membershipStopped.status ? `reads "${membershipStopped.status}"` : 'ended' }}.
+            It starts again on its own if you resubscribe. Turn it back on to
+            keep checking anyway.
+          </span>
+        </v-tooltip>
+      </v-chip>
+      <v-chip
+        v-else-if="(source.consecutive_failures || 0) > 0"
         size="x-small" color="error" variant="tonal" label
       >{{ source.consecutive_failures }}
         <!-- #1: show the actual failure reason on hover instead of a bare count. -->
@@ -109,6 +126,12 @@ const props = defineProps({
 // operator's subscription, not anything FC can retry. The count is only joined
 // in by the list endpoint, so phrase it without one when it's absent rather
 // than rendering a fabricated zero.
+// #3995: set by the membership sweep when it stops a source whose paid access
+// ended; cleared when it resumes, or when the operator toggles the source.
+const membershipStopped = computed(
+  () => props.source.config_overrides?._membership_stopped || null,
+)
+
 const noAccessTip = computed(() => {
   const n = props.source.tier_gated_count
   const what = n