🐛 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/testserver/server.go

@@ -15,6 +15,7 @@ import (
 	"sync"
 	"time"
 
+	"dance-lessons-coach/pkg/cache"
 	"dance-lessons-coach/pkg/config"
 	"dance-lessons-coach/pkg/server"
 	"dance-lessons-coach/pkg/user"
@@ -48,6 +49,7 @@ type Server struct {
 	baseURL            string
 	db                 *sql.DB
 	authService        user.AuthService // Reference to auth service for cleanup
+	cacheService       cache.Service    // Reference to cache service for cleanup
 	schemaMutex        sync.Mutex       // Protects schema operations
 	currentSchema      string           // Current schema being used
 	originalSearchPath string           // Original search_path to restore
@@ -153,6 +155,9 @@ func (s *Server) Start() error {
 	// Store auth service for cleanup
 	s.authService = realServer.GetAuthService()
 
+	// Store cache service for cleanup
+	s.cacheService = realServer.GetCacheService()
+
 	// Initialize database connection for cleanup
 	if err := s.initDBConnection(); err != nil {
 		return fmt.Errorf("failed to initialize database connection: %w", err)
@@ -409,6 +414,23 @@ func (s *Server) ResetJWTSecrets() error {
 	return nil
 }
 
+// FlushCache clears all cached data to prevent cache pollution between scenarios
+// This prevents cached responses from affecting subsequent test scenarios
+func (s *Server) FlushCache() error {
+	if s.cacheService == nil {
+		if isCleanupLoggingEnabled() {
+			log.Info().Msg("CLEANUP: No cache service available, skipping cache flush")
+		}
+		return nil
+	}
+
+	s.cacheService.Flush()
+	if isCleanupLoggingEnabled() {
+		log.Info().Msg("CLEANUP: Cache flushed successfully")
+	}
+	return nil
+}
+
 // CleanupDatabase deletes all test data from all tables
 // This uses raw SQL to avoid dependency on repositories and handles foreign keys properly
 // Uses SET CONSTRAINTS ALL DEFERRED to temporarily disable foreign key checks
@@ -555,7 +577,7 @@ func (s *Server) SetupScenarioSchema(feature, scenario string) error {
 		return fmt.Errorf("failed to create schema %s: %w", schemaName, err)
 	}
 
-	// Set search path to use the new schema
+	// Set search path to use the new schema (testserver's own connection)
 	searchPathSQL := fmt.Sprintf("SET search_path = %s, %s", schemaName, s.originalSearchPath)
 	if _, err := s.db.Exec(searchPathSQL); err != nil {
 		return fmt.Errorf("failed to set search_path: %w", err)