🐛 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

pkg/bdd/suite.go

@@ -115,6 +115,15 @@ func InitializeTestSuite(ctx *godog.TestSuiteContext) {
 				testserver.TraceStateJWTSecretOperation(feature, scenarioKey, "RESET", "ok")
 			}
 
+			// Flush cache after every scenario to prevent cache pollution
+			if flushErr := sharedServer.FlushCache(); flushErr != nil {
+				if isCleanupLoggingEnabled() {
+					log.Warn().Err(flushErr).Msg("CLEANUP: Failed to flush cache after scenario")
+				}
+			} else {
+				testserver.TraceStateCacheOperation(feature, scenarioKey, "FLUSH", "ok")
+			}
+
 			// Clean database after every scenario (only if schema isolation is disabled)
 			if !isSchemaIsolationEnabled() {
 				if cleanupErr := sharedServer.CleanupDatabase(); cleanupErr != nil {