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/injector.go

@@ -1,22 +1,24 @@
 package engine
 
 import (
+	"context"
 	"fmt"
 	"log"
 	"strings"
 
+	"github.com/insertr/insertr/internal/db"
 	"golang.org/x/net/html"
 )
 
 // Injector handles content injection into HTML elements
 type Injector struct {
-	client       ContentClient
+	client       db.ContentRepository
 	siteID       string
 	authProvider *AuthProvider
 }
 
 // NewInjector creates a new content injector
-func NewInjector(client ContentClient, siteID string) *Injector {
+func NewInjector(client db.ContentRepository, siteID string) *Injector {
 	return &Injector{
 		client:       client,
 		siteID:       siteID,
@@ -25,7 +27,7 @@ func NewInjector(client ContentClient, siteID string) *Injector {
 }
 
 // NewInjectorWithAuth creates a new content injector with auth provider
-func NewInjectorWithAuth(client ContentClient, siteID string, authProvider *AuthProvider) *Injector {
+func NewInjectorWithAuth(client db.ContentRepository, siteID string, authProvider *AuthProvider) *Injector {
 	if authProvider == nil {
 		authProvider = &AuthProvider{Type: "mock"}
 	}
@@ -39,7 +41,7 @@ func NewInjectorWithAuth(client ContentClient, siteID string, authProvider *Auth
 // InjectContent replaces element content with database values and adds content IDs
 func (i *Injector) InjectContent(element *Element, contentID string) error {
 	// Fetch content from database/API
-	contentItem, err := i.client.GetContent(i.siteID, contentID)
+	contentItem, err := i.client.GetContent(context.Background(), i.siteID, contentID)
 	if err != nil {
 		return fmt.Errorf("fetching content for %s: %w", contentID, err)
 	}
@@ -68,7 +70,7 @@ func (i *Injector) InjectBulkContent(elements []ElementWithID) error {
 	}
 
 	// Bulk fetch content
-	contentMap, err := i.client.GetBulkContent(i.siteID, contentIDs)
+	contentMap, err := i.client.GetBulkContent(context.Background(), i.siteID, contentIDs)
 	if err != nil {
 		return fmt.Errorf("bulk fetching content: %w", err)
 	}