refactor(downloads): unify phase-2 dispatch in download_backends — A5 (plan #707)

Mirror verify_source_credential: download_backends.run_download is now the single
download entry, so that module is the ONE registry of how each platform both
downloads AND verifies — the seam that makes adding a platform a bounded job
(write its adapter construction next to its verify). The native-ingester
construction + campaign-id resolution moves out of download_service into
download_backends._run_native_ingester.

download_service.download_source drops its `if uses_native_ingester ... else
gdl.download` branch and calls one `self._run_download(...)` (a thin delegate to
run_download passing the service's gdl + sync sessionmaker). mode (tick/backfill/
recovery) is still chosen there from the backfill state machine and threaded
through. Removed the now-unused PatreonIngester / resolve_campaign_id_for_source
imports from download_service.

Tests: the phase-2 stub seam moves from svc._run_patreon_ingester to
svc._run_download (helper + the db-release test); the two native-construction
tests repoint to download_backends.run_download (patching
download_backends.resolve_campaign_id_for_source / PatreonIngester).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/download_backends.py

@@ -20,8 +20,13 @@ function regardless of platform:
 
 from __future__ import annotations
 
+import asyncio
 from pathlib import Path
 
+from .gallery_dl import DownloadResult, ErrorType
+from .patreon_ingester import PatreonIngester
+from .patreon_resolver import resolve_campaign_id_for_source
+
 # Platforms whose download + verify go through the native ingester rather than
 # gallery-dl. gallery-dl still serves every other platform (subscribestar,
 # hentaifoundry, discord, pixiv, deviantart) unchanged.
@@ -34,6 +39,111 @@ def uses_native_ingester(platform: str) -> bool:
     return platform in NATIVE_INGESTER_PLATFORMS
 
 
+async def run_download(
+    *,
+    ctx: dict,
+    source_config,
+    skip_value: bool | str,
+    mode: str | None,
+    gdl,
+    sync_session_factory,
+) -> tuple[DownloadResult, str | None]:
+    """Uniform download across backends — the download counterpart to
+    `verify_source_credential`, so this module is the ONE place that knows how
+    each platform both downloads AND verifies (the seam that makes adding a
+    platform a bounded job).
+
+    Returns `(DownloadResult, resolved_campaign_id)`; `resolved_campaign_id` is
+    non-None only when a native vanity lookup ran this call (so phase 3 caches
+    it). Native platforms route through their ingester in `mode`
+    (tick/backfill/recovery); gallery-dl platforms run the subprocess. The caller
+    (download_service) prepares `source_config`/`skip_value`/`mode` from the
+    backfill state machine and owns phase 3.
+    """
+    platform = ctx["platform"]
+    if uses_native_ingester(platform):
+        return await _run_native_ingester(
+            ctx, source_config, mode, gdl, sync_session_factory
+        )
+    result = await gdl.download(
+        url=ctx["url"],
+        artist_slug=ctx["artist_slug"],
+        platform=platform,
+        source_config=source_config,
+        cookies_path=ctx["cookies_path"],
+        auth_token=ctx["auth_token"],
+        skip_value=skip_value,
+    )
+    return result, None
+
+
+async def _run_native_ingester(
+    ctx: dict, source_config, mode: str | None, gdl, sync_session_factory,
+) -> tuple[DownloadResult, str | None]:
+    """Patreon (today the only native platform): resolve the campaign id, then run
+    the native ingester in a worker thread (it is sync requests/subprocess).
+
+    `resolved_campaign_id` is non-None only when we had to look it up from the
+    vanity URL this run, so phase 3 caches it the way the old gallery-dl retry
+    did. A campaign id we cannot resolve is a loud NOT_FOUND — never a silent
+    empty success.
+    """
+    overrides = ctx["config_overrides"] or {}
+    campaign_id, resolved_campaign_id = await resolve_campaign_id_for_source(
+        ctx["url"], ctx["cookies_path"], overrides
+    )
+    if not campaign_id:
+        return (
+            DownloadResult(
+                success=False,
+                url=ctx["url"],
+                artist_slug=ctx["artist_slug"],
+                platform="patreon",
+                error_type=ErrorType.NOT_FOUND,
+                error_message=(
+                    "Could not resolve Patreon campaign id from the source URL "
+                    "(vanity lookup failed — cookies expired or creator moved?)"
+                ),
+            ),
+            None,
+        )
+
+    # Honor the operator's existing rate-limit knobs on the native path (plan
+    # #703): the global download_rate_limit_seconds (gallery-dl's `rate_limit`,
+    # here on the gdl service) paces media downloads; page fetches use the
+    # per-source sleep_request override, else `max(0.5, rate_limit/4)` — the same
+    # API-pacing default gallery-dl applied as its `sleep-request`.
+    rate_limit = gdl._rate_limit
+    request_sleep = (
+        source_config.sleep_request
+        if source_config.sleep_request is not None
+        else max(0.5, rate_limit / 4)
+    )
+    ingester = PatreonIngester(
+        images_root=gdl.images_root,
+        cookies_path=ctx["cookies_path"],
+        session_factory=sync_session_factory,
+        validate=gdl._validate_files,
+        rate_limit=rate_limit,
+        request_sleep=request_sleep,
+    )
+    loop = asyncio.get_running_loop()
+    dl_result = await loop.run_in_executor(
+        None,
+        lambda: ingester.run(
+            source_id=ctx["source_id"],
+            campaign_id=campaign_id,
+            artist_slug=ctx["artist_slug"],
+            url=ctx["url"],
+            mode=mode,
+            resume_cursor=source_config.resume_cursor,
+            time_budget_seconds=source_config.timeout,
+            posts_base=int(overrides.get("_backfill_posts", 0)),
+        ),
+    )
+    return dl_result, resolved_campaign_id
+
+
 async def verify_source_credential(
     *,
     platform: str,