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/enhancer.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/insertr/insertr/internal/config"
+	"github.com/insertr/insertr/internal/db"
 	"github.com/insertr/insertr/internal/engine"
 )
 
@@ -30,7 +31,7 @@ type Enhancer struct {
 }
 
 // NewEnhancer creates a new HTML enhancer with unified pipeline
-func NewEnhancer(client engine.ContentClient, siteID string, config EnhancementConfig) *Enhancer {
+func NewEnhancer(client db.ContentRepository, siteID string, config EnhancementConfig) *Enhancer {
 	return &Enhancer{
 		engine:     engine.NewContentEngine(client),
 		discoverer: NewDiscoverer(),
@@ -40,7 +41,7 @@ func NewEnhancer(client engine.ContentClient, siteID string, config EnhancementC
 }
 
 // NewEnhancerWithAuth creates a new HTML enhancer with auth provider
-func NewEnhancerWithAuth(client engine.ContentClient, siteID string, config EnhancementConfig, authProvider *engine.AuthProvider) *Enhancer {
+func NewEnhancerWithAuth(client db.ContentRepository, siteID string, config EnhancementConfig, authProvider *engine.AuthProvider) *Enhancer {
 	return &Enhancer{
 		engine:     engine.NewContentEngineWithAuth(client, authProvider),
 		discoverer: NewDiscoverer(),
@@ -50,7 +51,7 @@ func NewEnhancerWithAuth(client engine.ContentClient, siteID string, config Enha
 }
 
 // NewDefaultEnhancer creates an enhancer with default configuration
-func NewDefaultEnhancer(client engine.ContentClient, siteID string) *Enhancer {
+func NewDefaultEnhancer(client db.ContentRepository, siteID string) *Enhancer {
 	defaultConfig := EnhancementConfig{
 		Discovery: DiscoveryConfig{
 			Enabled:    true,