refactor(dry-S1): hoist app/client test fixtures into conftest

Removed the app/client fixtures duplicated across 36 test files (two
variants: separate app + client(app), and a self-contained client() that
called create_app inline) and the now-unused create_app imports. Both
fixtures now live once in conftest.py. test_suggestions_bulk keeps its
import (builds the app inline in two tests); test_health drops its local
client + unused pytest_asyncio.

Net -415 lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_suggestions_bulk.py

@@ -1,5 +1,6 @@
 import pytest
 
+from backend.app import create_app
 from backend.app.models import ImageRecord, TagKind
 from backend.app.models.tag import image_tag
 from backend.app.services.ml.suggestions import SuggestionService
@@ -86,8 +87,7 @@ async def test_consensus_threshold_clamped_and_empty_for_no_ids(db):
 
 @pytest.mark.asyncio
 async def test_bulk_suggestions_route(db):
-    from backend.app import create_app
-
+    
     tags = TagService(db)
     await tags.find_or_create("sword", TagKind.general)
     a = _img("i" * 64, {"sword": {"category": "general", "confidence": 0.97}})
@@ -107,8 +107,7 @@ async def test_bulk_suggestions_route(db):
 
 @pytest.mark.asyncio
 async def test_bulk_suggestions_requires_ids(db):
-    from backend.app import create_app
-
+    
     app = create_app()
     async with app.test_client() as c:
         resp = await c.post("/api/suggestions/bulk", json={})