feat: instance administration, bootstrap admin, and managed-tier OAuth gate

Self-hosted instances are now email/password only — OAuth providers
(GitHub/Google/Discord/Microsoft) activate solely when PRIMORA_MANAGED=true,
reserved for the future hosted tier.

Sign-up lifecycle: the first public sign-up becomes the instance admin; after
bootstrap, sign-up closes by default. A better-auth databaseHook gates only
/sign-up/email, so admin-created users and OAuth flows bypass it. The login
page reads GET /instance/public to hide the sign-up tab, show the bootstrap
notice, and render only the providers the instance offers.

Instance settings live in core.settings (migration 00010) and resolve
in-app > env > built-in default, cached 15s. Platform admins (better-auth
user.role) manage sign-up, mail transport, and rate limits via
GET/PUT/DELETE /instance/settings or the new Settings → Instance tab. Secret
values are AES-256-GCM encrypted at rest with PRIMORA_ENCRYPTION_KEY, never
returned by the API, and decrypted by the auth service at send time.

Auth service reads the same settings table for the sign-up gate and mail
config, so in-app changes take effect without a restart.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/app/app.go

@@ -92,6 +92,7 @@ func Bootstrap(ctx context.Context) (*App, error) {
 		return nil, fmt.Errorf("encryption key: %w", err)
 	}
 	platform := services.NewPlatformService(repo, store, services.NewMailer(cfg), os.Getenv("VITE_APP_URL"), dbxClient, encryptor, logger)
+	settings := services.NewSettingsService(repo, encryptor)
 
 	if cfg.Env == "production" {
 		gin.SetMode(gin.ReleaseMode)
@@ -126,10 +127,12 @@ func Bootstrap(ctx context.Context) (*App, error) {
 			APIKeyPerMinute: cfg.APIKeyRateLimitPerMin,
 			UserPerMinute:   cfg.UserRateLimitPerMin,
 		},
+		RateLimitResolver: settings.RateLimitPerMinute,
 	}.ResolveActor())
 
 	handler := &handlers.HTTPHandler{
 		Platform: platform,
+		Settings: settings,
 		Validate: validator.New(),
 		Metrics:  metrics,
 		Readiness: func(c *gin.Context) map[string]any {