feat: Discord on the native core ingester — client, downloader, ledgers, wiring (milestone 428)

Discord was the last focus platform still on gallery-dl. This adds the native
path, mirrored from gallery-dl 1.32.13's discord extractor:

- discord_client: API v10 with the user token and gallery-dl's request
  profile (dated Firefox UA, Referer). Walks a server, category, forum,
  channel or thread in gallery-dl's order and pages each channel newest-first.
  Files are attachments, then embeds, then forwards, numbered across the
  message. The resume cursor is <channel>:<before>. Text-only messages are
  not posts, since gallery-dl never made them.
- discord_downloader: gallery-dl's on-disk layout, cleaned the way it cleans
  names on Linux (only `/` and control characters change), so existing files
  are skipped_disk rather than fetched again. Sidecars carry identity only.
  The message record keeps gallery-dl's keys, so parse_sidecar,
  derive_post_url and the drop grouping read it unchanged.
- The ledger keys on the attachment id (or a hash of an embed's URL path),
  not the file's position, which an edit can renumber. Migration 0111.
- DiscordIngester: token auth, body canary off (files-only drops are
  normal). Registered as native, verified by token, and serialised
  per-platform, since every source shares one user token.
- ingest_core: optional `skip_feed` client seam (#4413). A tick's early-out
  on a multi-channel source now ends the quiet channel, not the whole walk.
  Clients without the seam behave as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/ingest_core.py

@@ -264,6 +264,14 @@ class Ingester:
         # operator-driven "re-read every body" pass; a horizon there would be a
         # third overlapping answer to a question that has two.
         post_meta = getattr(self.client, "post_meta", None)
+        # #4413: optional client seam for a source that is several feeds walked
+        # one after another (a Discord server: every channel and thread). The
+        # tick early-out means "this feed has nothing new", and without the
+        # seam it ends the WHOLE walk — so the first quiet channel would hide
+        # every channel after it. With it, the early-out asks the client to
+        # move on and the walk continues. Absent → the early-out ends the walk,
+        # exactly as before (Patreon and SubscribeStar are one feed each).
+        skip_feed = getattr(self.client, "skip_feed", None)
         horizon: datetime | None = None
         if mode == "tick" and revisit_days > 0 and post_meta is not None:
             horizon = datetime.now(UTC) - timedelta(days=revisit_days)
@@ -312,6 +320,7 @@ class Ingester:
         reached_bottom = False
         budget_hit = False
         early_out = False
+        feeds_caught_up = 0  # #4413: feeds a tick left early via skip_feed
         stopped = False  # plan #708 B4: operator hit Stop mid-walk
         cancel_armed = False  # latched once we observe a live "running" state
 
@@ -674,9 +683,15 @@ class Ingester:
                     })
 
                 if early_out:
-                    break
+                    if skip_feed is None:
+                        break
+                    skip_feed()
+                    feeds_caught_up += 1
+                    early_out = False
+                    consecutive_seen = 0
             else:
-                reached_bottom = True
+                # A walk that left feeds early did not read to their ends.
+                reached_bottom = not feeds_caught_up
         except self._error_base as exc:
             # The platform's client-error base — _failure_result (adapter)
             # maps it to a typed error.
@@ -724,6 +739,7 @@ class Ingester:
                 f", {revisited} post(s) updated ({revisit_downloads} new file(s))"
                 if revisited else ""
             )
+            + (f", {feeds_caught_up} feed(s) caught up" if feeds_caught_up else "")
             + (", reached end" if reached_bottom else "")
             + (", time-boxed" if budget_hit else "")
         )