Fix list_persons N+1 (the ~4s person-page load)

Opening any person page on a large tree took 4-5s on an idle server. Root cause:
list_persons looped over every person calling privacy.person_visibility (which
issues TWO get_membership_role queries per call) AND _attach_primary_name (one
name query per person). On the reporter's 2,324-person tree that's ~7,000
serialized DB round-trips per page load — the person page fetches the full
person list to build its name-lookup map.

Fix:
- Resolve the viewer's membership role ONCE. Members see the whole tree (full),
  so skip the per-person privacy engine entirely.
- Add _attach_primary_names: one batched names query (person_id IN (...),
  ordered the same as the single-person query so it picks the same name) instead
  of one per person.
- Apply the same batching to the non-member path, search_persons, the deleted-
  persons list, and public_view_service.list_public_persons.

Member-path list_persons goes from ~3·N queries to ~3 total. Other tree-wide
list endpoints (events/relationships/media/citations) were already flat selects.

Adds a regression test that asserts list_persons issues a constant number of
queries (not proportional to person count). Suite: 103 passing.

Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/services/public_view_service.py

@@ -33,7 +33,11 @@ from app.models.source import Citation, Source
 from app.models.tree import Tree
 from app.services import privacy
 from app.services.exceptions import NotFound
-from app.services.person_service import _attach_primary_name, _redact
+from app.services.person_service import (
+    _attach_primary_name,
+    _attach_primary_names,
+    _redact,
+)
 from app.services.privacy import Visibility
 
 
@@ -78,6 +82,7 @@ async def list_public_persons(
     session: AsyncSession, *, viewer_id: uuid.UUID | None, tree: Tree
 ) -> list[Person]:
     out: list[Person] = []
+    full: list[Person] = []
     for p in await _persons(session, tree):
         vis = await privacy.person_visibility(session, user_id=viewer_id, tree=tree, person=p)
         if vis == Visibility.hidden:
@@ -85,8 +90,9 @@ async def list_public_persons(
         if vis == Visibility.redacted:
             _redact(p)
         else:
-            await _attach_primary_name(session, p)
+            full.append(p)
         out.append(p)
+    await _attach_primary_names(session, full)  # one query, not one per person
     return out