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/engine/utils.go

@@ -1,7 +1,6 @@
 package engine
 
 import (
-	"database/sql"
 	"strings"
 
 	"golang.org/x/net/html"
@@ -38,7 +37,6 @@ func getAttribute(node *html.Node, key string) string {
 	return ""
 }
 
-
 // hasOnlyTextContent checks if a node contains only text content (no nested HTML elements)
 // DEPRECATED: Use hasEditableContent for more sophisticated detection
 func hasOnlyTextContent(node *html.Node) bool {
@@ -303,7 +301,6 @@ func hasInsertrClass(node *html.Node) bool {
 	return false
 }
 
-
 // isSelfClosing checks if an element is typically self-closing
 func isSelfClosing(node *html.Node) bool {
 	if node.Type != html.ElementNode {
@@ -376,24 +373,6 @@ func FindViableChildren(node *html.Node) []*html.Node {
 	return findViableChildren(node)
 }
 
-// SQL utility functions for consistent null string handling
-
-// ToNullString converts a string to sql.NullString
-func ToNullString(s string) sql.NullString {
-	if s == "" {
-		return sql.NullString{Valid: false}
-	}
-	return sql.NullString{String: s, Valid: true}
-}
-
-// FromNullString converts sql.NullString to string
-func FromNullString(ns sql.NullString) string {
-	if ns.Valid {
-		return ns.String
-	}
-	return ""
-}
-
 // Text extraction utility functions
 
 // ExtractTextContent extracts all text content from an HTML node recursively