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

tests/test_api_admin.py

@@ -390,6 +390,72 @@ async def test_prune_unused_commit_deletes_and_returns_count(client, db):
     assert "byebye" in body["sample_names"]
 
 
+# --- Tier-A: POST /posts/refetch-external ----------------------------
+
+
+@pytest.mark.asyncio
+async def test_refetch_external_resets_and_dispatches(client, db, monkeypatch):
+    """Surgical recovery for deep posts (2026-07-03): settled links reset to
+    pending with a fresh budget + dispatched; in-flight links untouched."""
+    from sqlalchemy import select
+
+    from backend.app.models import ExternalLink, Post, Source
+
+    a = Artist(name="Deep", slug="deep")
+    db.add(a)
+    await db.flush()
+    src = Source(artist_id=a.id, platform="patreon",
+                 url="https://www.patreon.com/deep", enabled=True)
+    db.add(src)
+    await db.flush()
+    post = Post(artist_id=a.id, source_id=src.id, external_post_id="96376640")
+    db.add(post)
+    await db.flush()
+    done = ExternalLink(post_id=post.id, artist_id=a.id, host="mega",
+                        url="https://mega.nz/file/a#k1", status="downloaded",
+                        attempts=1)
+    dead = ExternalLink(post_id=post.id, artist_id=a.id, host="gdrive",
+                        url="https://drive.google.com/x", status="dead",
+                        attempts=5, last_error="quota")
+    busy = ExternalLink(post_id=post.id, artist_id=a.id, host="mega",
+                        url="https://mega.nz/file/b#k2", status="downloading",
+                        attempts=1)
+    db.add_all([done, dead, busy])
+    await db.flush()
+    ids = (done.id, dead.id, busy.id)
+    await db.commit()
+
+    fetches = []
+    monkeypatch.setattr(
+        "backend.app.tasks.external.fetch_external_link.delay",
+        lambda lid: fetches.append(lid),
+    )
+
+    resp = await client.post(
+        "/api/admin/posts/refetch-external",
+        json={"external_post_id": "96376640"},
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert list(body["posts"].values()) == [{"links_total": 3, "links_reset": 2}]
+    assert sorted(fetches) == sorted([ids[0], ids[1]])
+
+    rows = dict((await db.execute(
+        select(ExternalLink.id, ExternalLink.status)
+        .where(ExternalLink.post_id == post.id)
+    )).all())
+    assert rows[ids[0]] == "pending"
+    assert rows[ids[1]] == "pending"
+    assert rows[ids[2]] == "downloading"     # in-flight left alone
+
+    # Unknown post → 404, nothing dispatched.
+    miss = await client.post(
+        "/api/admin/posts/refetch-external",
+        json={"external_post_id": "nope"},
+    )
+    assert miss.status_code == 404
+
+
 # --- Tier-A: POST /posts/prune-bare + /posts/reconcile-duplicates ---
 # These two routes share _run_dry_run_op with the tag prunes (DRY pass,
 # task #753); cover their apply path + reconcile's source_id passthrough so