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)

backend/app/services/download_service.py

@@ -25,7 +25,14 @@ from sqlalchemy.orm import joinedload
 
 from ..models import Artist, DownloadEvent, Source
 from .credential_service import CredentialService
-from .gallery_dl import GalleryDLService, SourceConfig
+from .gallery_dl import (
+    BACKFILL_SKIP_VALUE,
+    BACKFILL_TIMEOUT_SECONDS,
+    TICK_SKIP_VALUE,
+    ErrorType,
+    GalleryDLService,
+    SourceConfig,
+)
 from .importer import Importer
 from .patreon_resolver import resolve_campaign_id
 from .scheduler_service import set_platform_cooldown
@@ -84,6 +91,18 @@ class DownloadService:
         ctx = setup
 
         source_config = SourceConfig.from_dict(ctx["config_overrides"] or {})
+        # alembic 0031 / plan #544: derive skip_value + timeout from the
+        # source's backfill_runs_remaining counter. When > 0, walk the full
+        # post history (skip: True + 1800s); when 0, exit gallery-dl after
+        # 20 contiguous archived items (skip: "exit:20" + the default
+        # 870s). Operator sets backfill via POST /api/sources/{id}/backfill.
+        backfill_remaining = ctx.get("backfill_runs_remaining", 0) or 0
+        if backfill_remaining > 0:
+            skip_value: bool | str = BACKFILL_SKIP_VALUE
+            source_config.timeout = BACKFILL_TIMEOUT_SECONDS
+        else:
+            skip_value = TICK_SKIP_VALUE
+
         effective_url = _effective_url(
             ctx["platform"], ctx["url"], ctx["config_overrides"] or {}
         )
@@ -95,6 +114,7 @@ class DownloadService:
             source_config=source_config,
             cookies_path=ctx["cookies_path"],
             auth_token=ctx["auth_token"],
+            skip_value=skip_value,
         )
 
         resolved_campaign_id: str | None = None
@@ -121,6 +141,7 @@ class DownloadService:
                         source_config=source_config,
                         cookies_path=ctx["cookies_path"],
                         auth_token=ctx["auth_token"],
+                        skip_value=skip_value,
                     )
 
         return await self._phase3_persist(
@@ -185,6 +206,7 @@ class DownloadService:
             "config_overrides": dict(source.config_overrides or {}),
             "cookies_path": cookies_path,
             "auth_token": auth_token,
+            "backfill_runs_remaining": source.backfill_runs_remaining or 0,
         }
 
     async def _phase3_persist(
@@ -259,12 +281,21 @@ class DownloadService:
         run_stats["quarantined_count"] = dl_result.files_quarantined
         stderr_summary = self.gdl._extract_errors_warnings(dl_result.stderr)
 
-        status = "ok" if (dl_result.success and import_summary["errors"] == 0) else "error"
+        # Plan #544: PARTIAL means the run downloaded ≥1 file but the
+        # subprocess didn't finish in budget (typically wall-clock timeout
+        # mid-walk). Real work happened; the next tick continues via
+        # gallery-dl's archive. NOT a failure for status purposes.
+        if dl_result.success and import_summary["errors"] == 0:
+            status = "ok"
+        elif dl_result.error_type == ErrorType.PARTIAL and import_summary["errors"] == 0:
+            status = "ok"
+        else:
+            status = "error"
         ev.status = status
         ev.finished_at = datetime.now(UTC)
         ev.files_count = import_summary["attached"]
         ev.bytes_downloaded = bytes_downloaded
-        ev.error = dl_result.error_message if not dl_result.success else None
+        ev.error = dl_result.error_message if status == "error" else None
         ev.metadata_ = {
             "run_stats": run_stats,
             "error_type": dl_result.error_type.value if dl_result.error_type else None,
@@ -279,6 +310,19 @@ class DownloadService:
             source_id=ctx["source_id"], status=status, error_message=ev.error,
             error_type=dl_result.error_type.value if dl_result.error_type else None,
         )
+        # Plan #544: backfill lifecycle — auto-complete when a clean
+        # backfill run drained the queue (gallery-dl exited 0 + zero files
+        # downloaded means there was nothing to fetch); otherwise decrement
+        # the counter. Next tick falls back to tick mode once it hits 0.
+        backfill_remaining = ctx.get("backfill_runs_remaining", 0) or 0
+        if backfill_remaining > 0:
+            src = (await self.async_session.execute(
+                select(Source).where(Source.id == ctx["source_id"])
+            )).scalar_one()
+            if dl_result.return_code == 0 and dl_result.files_downloaded == 0:
+                src.backfill_runs_remaining = 0
+            else:
+                src.backfill_runs_remaining = max(0, backfill_remaining - 1)
         await self.async_session.commit()
         return event_id