Refactor database layer to eliminate type switching and simplify architecture

- Replace type switching with clean repository pattern using sqlc-generated code
- Move ContentRepository interface and domain models to db package
- Create separate SQLiteRepository and PostgreSQLRepository implementations
- Remove unnecessary RepositoryAdapter and ContentClient interface duplication
- Update all clients (HTTP, Mock) to implement db.ContentRepository directly
- Add context.Context parameters to all repository methods (Go best practice)
- Eliminate duplicate domain models and type conversions
- Remove type aliases - use db package types directly throughout codebase
- Update engine, content managers, and API handlers to use repositories directly

Benefits:
- Zero runtime type switching overhead
- Single source of truth for domain models
- Clean package boundaries and separation of concerns
- Standard Go interface patterns with context support
- Easier testing with mockable repository interface
- Maintainable: adding new database types requires no changes to existing code

cmd/enhance.go

@@ -11,7 +11,6 @@ import (
 
 	"github.com/insertr/insertr/internal/content"
 	"github.com/insertr/insertr/internal/db"
-	"github.com/insertr/insertr/internal/engine"
 )
 
 var enhanceCmd = &cobra.Command{
@@ -93,7 +92,7 @@ func runEnhance(cmd *cobra.Command, args []string) {
 	}
 
 	// Create content client
-	var client engine.ContentClient
+	var client db.ContentRepository
 	if cfg.API.URL != "" {
 		fmt.Printf("🌐 Using content API: %s\n", cfg.API.URL)
 		client = content.NewHTTPClient(cfg.API.URL, cfg.API.Key)
@@ -104,7 +103,7 @@ func runEnhance(cmd *cobra.Command, args []string) {
 			log.Fatalf("Failed to initialize database: %v", err)
 		}
 		defer database.Close()
-		client = engine.NewDatabaseClient(database)
+		client = database.NewContentRepository()
 	} else {
 		fmt.Printf("🧪 No database or API configured, using mock content\n")
 		client = content.NewMockClient()