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/serve.go

@@ -102,7 +102,7 @@ func runServe(cmd *cobra.Command, args []string) {
 	}
 
 	// Initialize content client for site manager
-	contentClient := engine.NewDatabaseClient(database)
+	contentClient := database.NewContentRepository()
 
 	// Initialize site manager with auth provider
 	authProvider := &engine.AuthProvider{Type: cfg.Auth.Provider}