fix(amp): enable OAuth fallback for Gemini v1beta1 routes

AMP CLI sends Gemini requests to non-standard paths that were being
directly proxied to ampcode.com without checking for local OAuth.

This fix adds:
- GeminiBridge handler to transform AMP CLI paths to standard format
- Enhanced model extraction from AMP's /publishers/google/models/* paths
- FallbackHandler wrapper to check for local OAuth before proxying

Flow:
- If user has local Google OAuth → use it (free tier)
- If no local OAuth → fallback to ampcode.com (charges credits)

Fixes issue where gemini-3-pro-preview requests always charged AMP
credits even when user had valid Google Cloud OAuth configured.

internal/api/modules/amp/amp.go

@@ -131,7 +131,7 @@ func (m *AmpModule) Register(ctx modules.Context) error {
 		// Register management proxy routes (requires upstream)
 		// Restrict to localhost by default for security (prevents drive-by browser attacks)
 		handler := proxyHandler(proxy)
-		m.registerManagementRoutes(ctx.Engine, handler, ctx.Config.AmpRestrictManagementToLocalhost)
+		m.registerManagementRoutes(ctx.Engine, ctx.BaseHandler, handler, ctx.Config.AmpRestrictManagementToLocalhost)
 
 		log.Infof("Amp upstream proxy enabled for: %s", upstreamURL)
 		log.Debug("Amp provider alias routes registered")