refactor(e5): deferred review-tail (backend) — enrich off request path, reuse, tests

Address the deep-review's deferred backend items so E5 is review-complete:

- person-enrich OFF the request path (ABUSE finding): person_detail no longer makes
  the TMDB call inline (even bounded to 8s it sat on the serve thread, un-throttled,
  demo-reachable — it violated the module's "no live TMDB call at serve time"
  invariant). It now schedules a FastAPI BackgroundTask (people._enrich_person_bg,
  fresh session, best-effort) after the response; the first open serves the row as-is
  and the bio lands by the next open. The inline try/rollback recovery is gone with it.

- reuse: a shared tmdb.resolve_library_guids owns the collision-safe guid→library
  bucketing (movie vs tv ids are independent sequences) that resolve_recs and
  people._resolve_known_for each hand-duplicated; a new rollup.item_state_map dedups
  the "PlexState-by-item_id" fetch across the person + discover paths.

- test gaps: enrich_batch transient TmdbError leaves the watermark NULL to retry;
  guid_to_tmdb_id resolves a tvdb source + tv media_type via /find's tv_results;
  _continue_watching includes a NULL-status started movie and excludes a hidden one;
  /api/search/youtube returns 422 (not 5xx) when ingest fails (route-level, B-core-1).

Backend lane 403 → 413, ruff clean. No migration; serve contracts unchanged.

backend/tests/test_plex_person_bio.py

@@ -100,34 +100,51 @@ def _get(db, user, monkeypatch, path):
         app.dependency_overrides.clear()
 
 
-def test_person_detail_survives_poisoned_enrich(db, monkeypatch):
-    # If the lazy enrich_person leaves the session in a failed-transaction state (e.g. a commit-time
-    # flush error), person_detail's except must roll it back so the following SELECTs still run — the
-    # page renders despite the enrich failure ("never fail the page"). Without the db.rollback() fix the
-    # poisoned session would raise PendingRollbackError on the next query and 500.
-    from sqlalchemy import text
+def test_person_detail_schedules_enrich_off_request_path(db, monkeypatch):
+    # The bio fetch runs OFF the request path (a background task after the response), so the serve never
+    # makes a live TMDB call. An un-enriched person (tmdb_person_id set + tmdb_enriched_at None) schedules
+    # the enrich and the page still renders immediately with whatever's in the row (bio still null).
+    from app.routes.plex import people
 
     lib = PlexLibrary(plex_key="9", title="Movies", kind="movie", enabled=True)
     db.add(lib)
     db.flush()
-    # Un-enriched person (tmdb_person_id set + tmdb_enriched_at None) → triggers the lazy enrich path.
-    db.add(PlexPerson(name="Poison P", tmdb_person_id=777))
-    user = User(email="poison@example.com")
+    db.add(PlexPerson(name="Lazy P", tmdb_person_id=777))
+    user = User(email="lazybg@example.com")
     db.add(user)
     db.commit()
 
-    def _poison_then_raise(db_, person_):
-        try:
-            db_.execute(text("SELECT 1 / 0"))  # aborts the transaction (like a failed enrich commit)
-        except Exception:  # noqa: BLE001 — swallow so the tx stays poisoned, then raise like the real path
-            pass
-        raise RuntimeError("enrich commit blew up")
+    scheduled: list[int] = []
+    monkeypatch.setattr(people, "_enrich_person_bg", lambda pid: scheduled.append(pid))
 
-    monkeypatch.setattr(plex_tmdb, "enrich_person", _poison_then_raise)
+    resp = _get(db, user, monkeypatch, "/api/plex/person/Lazy P")
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["biography"] is None  # not enriched yet — the bio lands by the next open
+    assert len(scheduled) == 1  # the enrich was scheduled off the request path
 
-    resp = _get(db, user, monkeypatch, "/api/plex/person/Poison P")
-    assert resp.status_code == 200, resp.text  # page still renders (session was rolled back)
-    assert resp.json()["biography"] is None  # enrich failed, so no bio
+
+def test_person_detail_no_enrich_when_already_enriched(db, monkeypatch):
+    # An already-enriched person (tmdb_enriched_at set) must NOT schedule another background fetch.
+    from datetime import datetime, timezone
+
+    from app.routes.plex import people
+
+    lib = PlexLibrary(plex_key="8", title="Movies", kind="movie", enabled=True)
+    db.add(lib)
+    db.flush()
+    db.add(PlexPerson(name="Done P", tmdb_person_id=888,
+                      tmdb_enriched_at=datetime(2024, 1, 1, tzinfo=timezone.utc), biography="Bio."))
+    user = User(email="donebg@example.com")
+    db.add(user)
+    db.commit()
+
+    scheduled: list[int] = []
+    monkeypatch.setattr(people, "_enrich_person_bg", lambda pid: scheduled.append(pid))
+
+    resp = _get(db, user, monkeypatch, "/api/plex/person/Done P")
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["biography"] == "Bio."
+    assert scheduled == []  # already enriched → no background work
 
 
 def test_person_detail_serves_bio_and_resolves_known_for(db, monkeypatch):