test(backend): add 55+ test files to push coverage to 98%

Subagents delivered tests across all uncovered backend modules:

Schemas (100%): agent, auth, health, profile, settings, validation
Services (98-100%): auth, profile, health, llm, mapping, resource,
  security, git, superset_lookup, sql_table_extractor, rbac
API routes (new): auth, admin, health, environments, plugins,
  dashboards (helpers, projection, actions, listing),
  git (config, deps, env, helpers)
Clean Release (100%): DTO, facade, policy_engine, stages,
  repos, preparation, source_isolation, compliance
Git services: base, remote_providers
Agent module: app, run, middleware, langgraph_setup
Core: trace, cleanup, ws_log_handler, timezone, auth (config/oauth/security), matching
Reports: normalizer, report_service, type_profiles
Notifications: service, providers

Also:
- .gitignore: add .coverage, *.cover, coverage-* dirs
- src/schemas/auth.py: fix AD group DN regex (comma in CN=...)
- Remove co-located src/services/__tests__/ (caused pytest module collision)

.gitignore

@@ -76,7 +76,12 @@ backend/:memory
 node_modules/
 .venv/
 coverage/
+coverage-summary/
 *.tmp
+.coverage
+*.cover
+coverage_html_backend/
+coverage_html_frontend/
 audit_report.txt
 check_semantics.py
 docs_audit_report.txt