feat: Google Photos provider backend + API hardening

- Add Google Photos provider: client, models, change detector, capabilities
- Add notification templates (en/ru) for all GP event slots
- Add command templates (en/ru) for GP bot commands
- Register GP in slot/command loaders, capabilities, and seeds
- Harden provider API: validate OAuth credentials on create/update
- Add internal URL rewriting for asset fetches (LAN optimization)
- Fix template renderer to handle missing variables gracefully
- Improve webhook command routing for multi-provider support
- Add provider health check endpoint and watcher improvements

frontend/src/routes/providers/+page.svelte

@@ -56,8 +56,8 @@
 		} catch (err: any) {
 			loadError = err.message || t('providers.loadError');
 		} finally { loaded = true; highlightFromUrl(); }
-		// Ping all providers in background
-		for (const p of providers) {
+		// Ping all providers in background (use unfiltered list)
+		for (const p of allProviders) {
 			health = { ...health, [p.id]: null };
 			api(`/providers/${p.id}/test`, { method: 'POST' })
 				.then((r: any) => { health = { ...health, [p.id]: r.ok }; })