feat: add Scheduler provider + multi-provider UX fixes

Scheduler provider:
- Virtual provider (no external service) that emits SCHEDULED_MESSAGE
  events on user-defined intervals or cron expressions
- Custom variables stored in tracker filters, flattened into template context
- fire_count persists across triggers via tracker state
- APScheduler CronTrigger support for cron-mode schedules
- Default templates (EN+RU), seeded on startup

Multi-provider UX fixes:
- Tracking config hides Immich-specific sections (periodic, scheduled,
  memory, asset display) for non-Immich providers
- Command config driven by provider capabilities — hides commands/settings
  for providers without bot commands
- Template config hides empty "Scheduled Messages" group
- Test menu on tracker targets is provider-aware (Immich shows all 4 test
  types, others show only basic)
- Removed redundant Test button from tracker card
- System-owned tracking configs (user_id=0) seeded for Gitea + Scheduler
- Fixed ownership checks to allow system configs in tracker-target links
- Capabilities cache shared across template-configs and command-configs
- Command tracker bot selector uses EntitySelect instead of raw select
- Sample context includes Gitea + Scheduler variables for template preview

packages/server/src/notify_bridge_server/api/providers.py

@@ -97,6 +97,8 @@ async def create_provider(
                         detail=test_result.get("message", "Cannot connect to Gitea"),
                     )
 
+    # Scheduler: no validation needed (virtual provider)
+
     provider = ServiceProvider(
         user_id=user.id,
         type=body.type,
@@ -253,6 +255,9 @@ async def test_provider(
             gitea = make_gitea_provider(http_session, provider)
             return await gitea.test_connection()
 
+    if provider.type == "scheduler":
+        return {"ok": True, "message": "Virtual provider — always available"}
+
     return {"ok": False, "message": f"Unknown provider type: {provider.type}"}