feat(fc2a): add TagService — find-or-create, autocomplete, image association

Validates kind/fandom rules at the service layer: fandom_id only allowed
for kind='character', and must reference an existing kind='fandom' tag.
Autocomplete ranking: exact match > prefix match > substring, tie-broken
by image_count descending. Image-tag association uses INSERT ON CONFLICT
DO NOTHING for idempotent re-tagging.

conftest.py adds a transactional AsyncSession fixture; each test rolls
back so they don't pollute each other. Also includes a sync Session
fixture (db_sync) for the Importer tests in Task 5.

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

backend/app/services/__init__.py

@@ -0,0 +1 @@
+"""Business-logic services consumed by API blueprints and Celery tasks."""