feat(recovery): surgical re-fetch for deep posts via ExternalLink reset

Operator-flagged: the recovered defective files live DEEP in their artists'
back-catalogues — the normal download cadence (by design, via the seen-gates)
will never re-walk them, so recovery's source re-check alone can't bring them
back. The durable per-post handle is the ExternalLink row, which survives the
image delete:

- services/external_links.refetch_links_for_post: reset settled links to
  pending (fresh attempt budget, in-flight left alone) + dispatch their
  fetches; sha-dedupe at import discards payload files that still exist, so
  only the missing file lands.
- recover_defective_image now captures the image's post ids BEFORE the delete
  cascades provenance away and resets those posts' links — future recoveries
  are surgical automatically (response gains links_reset; source re-check
  stays for gallery-dl-native files within walk reach).
- POST /api/admin/posts/refetch-external {external_post_id, source_id?} — the
  manual tool for the three files recovered before this fix existed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/admin.py

@@ -7,6 +7,7 @@ Action surfaces:
   POST   /api/admin/tags/<int:dest_id>/merge       (Tier B)
   POST   /api/admin/tags/prune-unused              (Tier A)
   POST   /api/admin/posts/prune-bare               (Tier A)
+  POST   /api/admin/posts/refetch-external         (Tier A)
   GET    /api/admin/tags/<int:tag_id>/usage-count  (helper)
 
 Tier-C ops take a dry_run body flag (returns projection inline,
@@ -22,7 +23,7 @@ from quart import Blueprint, jsonify, request
 from sqlalchemy import select, text
 
 from ..extensions import get_session
-from ..models import Artist
+from ..models import Artist, Post
 from ..services.cleanup_service import project_artist_cascade, project_bulk_image_delete
 from ._responses import error_response as _bad
 
@@ -276,6 +277,44 @@ async def posts_reconcile_duplicates():
     return await _run_dry_run_op(reconcile_duplicate_posts, source_id=source_id)
 
 
+@admin_bp.route("/posts/refetch-external", methods=["POST"])
+async def posts_refetch_external():
+    """Surgical re-fetch of a post's external file-host links (operator
+    2026-07-03): the normal cadence never re-walks deep back-catalogue posts,
+    so a deleted external file only comes back by resetting its ExternalLink
+    row(s) — this endpoint does that per post and dispatches the fetches.
+    Sha-dedupe discards payload files that still exist, so only what's
+    missing lands. Body: {external_post_id: str, source_id?: int (to
+    disambiguate the same external id across sources)}."""
+    from ..services.external_links import refetch_links_for_post
+
+    body = await request.get_json(silent=True) or {}
+    ext_id = str(body.get("external_post_id") or "").strip()
+    if not ext_id:
+        return _bad("missing_external_post_id",
+                    detail="external_post_id is required")
+    raw_source = body.get("source_id")
+    try:
+        source_id = int(raw_source) if raw_source is not None else None
+    except (TypeError, ValueError):
+        return _bad("invalid_source_id", detail="source_id must be an integer")
+
+    async with get_session() as session:
+        stmt = select(Post.id).where(Post.external_post_id == ext_id)
+        if source_id is not None:
+            stmt = stmt.where(Post.source_id == source_id)
+        post_ids = (await session.execute(stmt)).scalars().all()
+        if not post_ids:
+            return _bad("post_not_found", status=404,
+                        detail=f"no post with external_post_id {ext_id!r}")
+        results = {}
+        for pid in post_ids:
+            results[str(pid)] = await session.run_sync(
+                lambda s, p=pid: refetch_links_for_post(s, p)
+            )
+    return jsonify({"posts": results})
+
+
 def _reset_content_confirm_token(projection: dict) -> str:
     """Stable 8-hex token derived from the live counts (mirrors the Tier-C
     bulk-delete token): it changes whenever the data changes, so the apply can