Phase 3: PDF/DOCX extraction, chunking, LLM client with mock interface

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -11,6 +11,7 @@ type Config struct {
 	DataDir       string
 	Port          string
 	AdminUsers    []AdminUser
+	LLMModel      string // defaults to gpt-4o-mini
 }
 
 type AdminUser struct {
@@ -24,6 +25,7 @@ func Load() *Config {
 		SessionSecret: os.Getenv("SESSION_SECRET"),
 		DataDir:       envOr("DATA_DIR", "./data"),
 		Port:          envOr("PORT", "8080"),
+		LLMModel:      envOr("LLM_MODEL", "gpt-4o-mini"),
 	}
 	cfg.AdminUsers = parseAdminUsers(os.Getenv("ADMIN_USERS"))
 	return cfg