🐛 fix(bdd): revert PR #26 schema isolation, add cache flush + sequential test execution

PR #26 added BDD_SCHEMA_ISOLATION=true to CI but this creates per-scenario schemas
WITHOUT running migrations on them, causing 500 errors on user registration. This
PR reverts that and instead relies on:

1. The existing CleanupDatabase hook (truncates all tables AfterScenario)
2. Sequential test package execution (-p 1) to avoid contention between feature
   packages sharing the same Postgres DB

Plus defensive additions for future-proofing:
- pkg/server/server.go: GetCacheService() exposed for test cleanup
- pkg/bdd/testserver/server.go: cacheService field + FlushCache() method
- pkg/bdd/testserver/state_tracer.go: TraceStateCacheOperation
- pkg/bdd/suite.go: AfterScenario hook calls FlushCache()
- scripts/run-bdd-tests.sh: -p 1 added (sequential package execution)

Validation:
- AuthBDD alone: 5/5 PASS (was 0/5 with broken schema isolation)
- Full features/ via run-bdd-tests.sh: ALL PASS (auth, config, greet, health, jwt)

Out of scope (follow-up T12):
- Proper parallel BDD with schema migrations per scenario + dedicated connection
  pools. Required for scaling tests but architecturally significant. Tracked.

Co-Authored-By: Mistral Vibe (devstral-2 / mistral-medium-3.5) - cache flush diagnosis
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com> - root cause + revert

.gitea/workflows/ci-cd.yaml

@@ -219,10 +219,10 @@ jobs:
           export DLC_DATABASE_PASSWORD=postgres
           export DLC_DATABASE_NAME=dance_lessons_coach_bdd_test
           export DLC_DATABASE_SSL_MODE=disable
-          # Enable per-scenario schema isolation (ADR-0025) to prevent flaky AuthBDD failures.
-          # Without this, scenarios share the public schema and pollute each other's state.
-          # Observed flakiness: same code passes in #605, fails in #606 on TestAuthBDD/*.
-          export BDD_SCHEMA_ISOLATION=true
+          # NOTE: BDD_SCHEMA_ISOLATION was tried (PR #26) but creates empty per-scenario schemas
+          # without table migrations, causing 500 errors on registration. Reverted in PR #28.
+          # The default mode (CleanupDatabase truncates between scenarios) works fine when tests
+          # run sequentially (Go BDD doesn't use t.Parallel by default).
           ./scripts/run-bdd-tests.sh
           
           # Generate BDD coverage report