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

internal/db/database.go

@@ -105,6 +105,18 @@ func (db *Database) GetPostgreSQLDB() *sql.DB {
 	return db.conn
 }
 
+// NewContentRepository creates a repository based on the database type
+func (db *Database) NewContentRepository() ContentRepository {
+	switch db.dbType {
+	case "sqlite3":
+		return NewSQLiteRepository(db.conn)
+	case "postgresql":
+		return NewPostgreSQLRepository(db.conn)
+	default:
+		panic(fmt.Sprintf("unsupported database type: %s", db.dbType))
+	}
+}
+
 // initializeSQLiteSchema sets up the SQLite database schema
 func (db *Database) initializeSQLiteSchema() error {
 	ctx := context.Background()