Refactor architecture: eliminate auto-discovery and consolidate packages

- Remove auto-discovery entirely (~450 lines)
  * Delete internal/content/discoverer.go
  * Simplify enhancer to single-phase processing
  * Remove duplicate container expansion logic

- Consolidate repository implementations
  * Move internal/content/client.go → internal/db/http_client.go
  * Group all repository implementations in db/ package

- Add file utilities to engine following Go stdlib patterns
  * Add engine.ProcessFile() and ProcessDirectory() methods
  * Engine now handles both content processing AND file operations

- Move site management to dedicated package
  * Move internal/content/site_manager.go → internal/sites/manager.go
  * Clear separation of site lifecycle from content processing

- Preserve container expansion (syntactic sugar)
  * .insertr on containers still auto-applies to viable children
  * Container detection logic consolidated in engine/utils.go

Result: Clean architecture with single source of truth for .insertr processing

internal/api/handlers.go

@@ -9,16 +9,16 @@ import (
 
 	"github.com/go-chi/chi/v5"
 	"github.com/insertr/insertr/internal/auth"
-	"github.com/insertr/insertr/internal/content"
 	"github.com/insertr/insertr/internal/db"
 	"github.com/insertr/insertr/internal/engine"
+	"github.com/insertr/insertr/internal/sites"
 )
 
 // ContentHandler handles all content-related HTTP requests
 type ContentHandler struct {
 	repository  db.ContentRepository
 	authService *auth.AuthService
-	siteManager *content.SiteManager
+	siteManager *sites.SiteManager
 	engine      *engine.ContentEngine
 }
 
@@ -36,7 +36,7 @@ func NewContentHandler(database *db.Database, authService *auth.AuthService) *Co
 }
 
 // SetSiteManager sets the site manager for file enhancement
-func (h *ContentHandler) SetSiteManager(siteManager *content.SiteManager) {
+func (h *ContentHandler) SetSiteManager(siteManager *sites.SiteManager) {
 	h.siteManager = siteManager
 }