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/content/site_manager.go

@@ -9,6 +9,7 @@ import (
 	"sync"
 
 	"github.com/insertr/insertr/internal/config"
+	"github.com/insertr/insertr/internal/db"
 	"github.com/insertr/insertr/internal/engine"
 )
 
@@ -21,12 +22,12 @@ type SiteManager struct {
 	enhancer      *Enhancer
 	mutex         sync.RWMutex
 	devMode       bool
-	contentClient engine.ContentClient
+	contentClient db.ContentRepository
 	authProvider  *engine.AuthProvider
 }
 
 // NewSiteManager creates a new site manager
-func NewSiteManager(contentClient engine.ContentClient, devMode bool) *SiteManager {
+func NewSiteManager(contentClient db.ContentRepository, devMode bool) *SiteManager {
 	return &SiteManager{
 		sites:         make(map[string]*SiteConfig),
 		enhancer:      NewDefaultEnhancer(contentClient, ""), // siteID will be set per operation
@@ -37,7 +38,7 @@ func NewSiteManager(contentClient engine.ContentClient, devMode bool) *SiteManag
 }
 
 // NewSiteManagerWithAuth creates a new site manager with auth provider
-func NewSiteManagerWithAuth(contentClient engine.ContentClient, devMode bool, authProvider *engine.AuthProvider) *SiteManager {
+func NewSiteManagerWithAuth(contentClient db.ContentRepository, devMode bool, authProvider *engine.AuthProvider) *SiteManager {
 	if authProvider == nil {
 		authProvider = &engine.AuthProvider{Type: "mock"}
 	}