feat(download): tick/backfill modes + partial-success classifier (plan #544)

Routine subscription polls walked the entire post history every tick
even when nothing had changed, because gallery-dl's default `skip: True`
continues iterating archived posts. A creator with ~550 archived posts
(Knuxy patreon) saturates the 870s wall-clock cap before completing,
even with zero downloads needed. Plus, a tier-limited run that
downloaded hundreds of files but ran out the clock should be a
warning, not an error.

Two coupled changes, both operator-flagged 2026-06-01:

* **Tick mode (default, cron polls).** New `TICK_SKIP_VALUE = "exit:20"`
  asks gallery-dl to exit after 20 contiguous archived items. Fresh
  subscriptions + new-content cases still walk normally; established
  subscription with zero new content exits in ~30s of HEAD requests
  instead of pegging the timeout. 20 (not 5) gives headroom against
  paywall warnings interleaving with archived items.
* **Backfill mode (explicit, operator-triggered).** Sticky for N runs
  via new `Source.backfill_runs_remaining` (alembic 0031). While > 0,
  downloads use `skip: True` + 1800s timeout. Auto-decrements per run
  with early-reset to 0 when a clean run finds zero files (queue
  drained). N defaults to 3 — multiple runs give the system enough
  budget to finish a deep walk across timeout boundaries. New
  `POST /api/sources/{id}/backfill` arms the source; "Deep scan"
  button on each SourceRow (chip shows remaining count) wires it.

Plus partial-success classifier: non-zero gallery-dl exit + ≥1 file
downloaded + no source-level error fires `ErrorType.PARTIAL`, which
download_service maps to `status=\"ok\"`. The run did real work; the
next tick continues via gallery-dl's archive. No more red events for
"timed out mid-walk after downloading 300 files."

Retires `SourceConfig.skip_existing` — skip value is now derived from
the source state and passed as a separate `skip_value` parameter
through download() / _build_config_for_source(). `GD_DEFAULTS` drops
the now-dead key (was inert data after this refactor).

Tests cover:
* tick + backfill skip-value emission in _build_config_for_source
* PARTIAL classifier branch + TIER_LIMITED-wins-over-PARTIAL ordering
* SourceService.set_backfill_runs validation + persistence
* /api/sources/{id}/backfill 200/400/404 paths
* download_service auto-decrement / auto-reset / tick-mode-no-touch
* PARTIAL → status=ok in the orchestrator (no consecutive_failures bump)

frontend/src/components/subscriptions/SourceRow.vue

@@ -31,6 +31,12 @@
         v-if="(source.consecutive_failures || 0) > 0"
         size="x-small" color="error" variant="tonal" label
       >{{ source.consecutive_failures }}</v-chip>
+      <v-chip
+        v-else-if="(source.backfill_runs_remaining || 0) > 0"
+        size="x-small" color="info" variant="tonal" label
+      >
+        backfill ({{ source.backfill_runs_remaining }}×)
+      </v-chip>
       <span v-else class="fc-source-row__zero">0</span>
     </td>
     <td class="fc-source-row__actions">
@@ -42,6 +48,16 @@
         <v-icon>mdi-play</v-icon>
         <v-tooltip activator="parent" location="top">Check now</v-tooltip>
       </v-btn>
+      <v-btn
+        icon="mdi-magnify-scan" size="x-small" variant="text"
+        :disabled="(source.backfill_runs_remaining || 0) > 0"
+        @click.stop="$emit('backfill', source)"
+      >
+        <v-icon>mdi-magnify-scan</v-icon>
+        <v-tooltip activator="parent" location="top">
+          Deep scan — walk full history for next few runs
+        </v-tooltip>
+      </v-btn>
       <v-btn
         icon="mdi-pencil" size="x-small" variant="text"
         @click.stop="$emit('edit', source)"
@@ -69,7 +85,7 @@ const props = defineProps({
   checking: { type: Boolean, default: false },
   warningThreshold: { type: Number, default: 5 },
 })
-const emit = defineEmits(['edit', 'remove', 'toggle', 'check'])
+const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill'])
 
 function onToggleEnabled(value) {
   emit('toggle', { source: props.source, enabled: value })