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/types.go

@@ -48,73 +48,3 @@ type ContentEntry struct {
 	HTMLContent string
 	Template    string
 }
-
-// ContentClient interface for accessing content data
-// This will be implemented by database clients, HTTP clients, and mock clients
-type ContentClient interface {
-	GetContent(siteID, contentID string) (*ContentItem, error)
-	GetBulkContent(siteID string, contentIDs []string) (map[string]ContentItem, error)
-	GetAllContent(siteID string) (map[string]ContentItem, error)
-	CreateContent(siteID, contentID, htmlContent, originalTemplate, lastEditedBy string) (*ContentItem, error)
-
-	// Collection operations
-	GetCollection(siteID, collectionID string) (*CollectionItem, error)
-	CreateCollection(siteID, collectionID, containerHTML, lastEditedBy string) (*CollectionItem, error)
-	GetCollectionItems(siteID, collectionID string) ([]CollectionItemWithTemplate, error)
-	GetCollectionTemplates(siteID, collectionID string) ([]CollectionTemplateItem, error)
-	CreateCollectionTemplate(siteID, collectionID, name, htmlTemplate string, isDefault bool) (*CollectionTemplateItem, error)
-	CreateCollectionItem(siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error)
-	CreateCollectionItemAtomic(siteID, collectionID string, templateID int, lastEditedBy string) (*CollectionItemWithTemplate, error)
-}
-
-// ContentItem represents a piece of content from the database
-type ContentItem struct {
-	ID               string `json:"id"`
-	SiteID           string `json:"site_id"`
-	HTMLContent      string `json:"html_content"`
-	OriginalTemplate string `json:"original_template"`
-	UpdatedAt        string `json:"updated_at"`
-	LastEditedBy     string `json:"last_edited_by,omitempty"`
-}
-
-// ContentResponse represents the API response structure
-type ContentResponse struct {
-	Content []ContentItem `json:"content"`
-	Error   string        `json:"error,omitempty"`
-}
-
-// CollectionItem represents a collection container from the database
-type CollectionItem struct {
-	ID            string `json:"id"`
-	SiteID        string `json:"site_id"`
-	ContainerHTML string `json:"container_html"`
-	UpdatedAt     string `json:"updated_at"`
-	LastEditedBy  string `json:"last_edited_by,omitempty"`
-}
-
-// CollectionTemplateItem represents a collection template from the database
-type CollectionTemplateItem struct {
-	TemplateID   int    `json:"template_id"`
-	CollectionID string `json:"collection_id"`
-	SiteID       string `json:"site_id"`
-	Name         string `json:"name"`
-	HTMLTemplate string `json:"html_template"`
-	IsDefault    bool   `json:"is_default"`
-}
-
-// CollectionItemWithTemplate represents a collection item with its template information
-type CollectionItemWithTemplate struct {
-	ItemID       string `json:"item_id"`
-	CollectionID string `json:"collection_id"`
-	SiteID       string `json:"site_id"`
-	TemplateID   int    `json:"template_id"`
-	HTMLContent  string `json:"html_content"`
-	Position     int    `json:"position"`
-	UpdatedAt    string `json:"updated_at"`
-	LastEditedBy string `json:"last_edited_by"`
-
-	// Template information
-	TemplateName string `json:"template_name"`
-	HTMLTemplate string `json:"html_template"`
-	IsDefault    bool   `json:"is_default"`
-}