feat: add target_lists table, schema, and CRUD endpoints

Introduces the TargetList model (team-scoped JSONB target arrays),
Pydantic schemas, and full CRUD REST API at /target-lists/.
Includes Alembic migration and 5 integration tests (TDD).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -22,6 +22,7 @@ from .account_limit_override import AccountLimitOverride
 from .feature_flag import FeatureFlag, PlanFeatureDefault, AccountFeatureOverride
 from .platform_setting import PlatformSetting
 from .user_pinned_tree import UserPinnedTree
+from .target_list import TargetList
 
 __all__ = [
     "User",
@@ -55,4 +56,5 @@ __all__ = [
     "AccountFeatureOverride",
     "PlatformSetting",
     "UserPinnedTree",
+    "TargetList",
 ]