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>

tests/conftest.py

@@ -0,0 +1,75 @@
+"""Shared pytest fixtures.
+
+The async db fixture provides an AsyncSession bound to a transaction that
+gets rolled back after each test. CI provisions a real Postgres + pgvector
+(see .forgejo/workflows/ci.yml), so tests exercise the actual schema and
+migration code paths.
+"""
+
+import os
+
+import pytest
+import pytest_asyncio
+from sqlalchemy import create_engine
+from sqlalchemy.ext.asyncio import (
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+from sqlalchemy.orm import sessionmaker
+
+
+def _async_database_url() -> str:
+    user = os.environ.get("DB_USER", "fabledcurator")
+    password = os.environ["DB_PASSWORD"]
+    host = os.environ.get("DB_HOST", "postgres")
+    port = os.environ.get("DB_PORT", "5432")
+    name = os.environ.get("DB_NAME", "fabledcurator_test")
+    return f"postgresql+asyncpg://{user}:{password}@{host}:{port}/{name}"
+
+
+def _sync_database_url() -> str:
+    user = os.environ.get("DB_USER", "fabledcurator")
+    password = os.environ["DB_PASSWORD"]
+    host = os.environ.get("DB_HOST", "postgres")
+    port = os.environ.get("DB_PORT", "5432")
+    name = os.environ.get("DB_NAME", "fabledcurator_test")
+    return f"postgresql+psycopg://{user}:{password}@{host}:{port}/{name}"
+
+
+@pytest_asyncio.fixture
+async def engine():
+    e = create_async_engine(_async_database_url(), future=True)
+    yield e
+    await e.dispose()
+
+
+@pytest_asyncio.fixture
+async def db(engine) -> AsyncSession:
+    Session = async_sessionmaker(engine, expire_on_commit=False)
+    async with Session() as session:
+        await session.begin()
+        try:
+            yield session
+        finally:
+            await session.rollback()
+
+
+@pytest.fixture
+def sync_engine():
+    e = create_engine(_sync_database_url(), future=True)
+    yield e
+    e.dispose()
+
+
+@pytest.fixture
+def db_sync(sync_engine):
+    """Synchronous Session bound to a savepoint — used by Importer tests
+    (the Importer is sync-only by design)."""
+    SyncSession = sessionmaker(sync_engine, expire_on_commit=False)
+    with SyncSession() as session:
+        session.begin()
+        try:
+            yield session
+        finally:
+            session.rollback()