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/webhooks.py

@@ -277,6 +277,8 @@ def _event_allowed_by_tracking_config(event: ServiceEvent, tc: TrackingConfig) -
         "pr_merged": tc.track_pr_merged,
         "pr_commented": tc.track_pr_commented,
         "release_published": tc.track_release_published,
+        # Scheduler events
+        "scheduled_message": tc.track_scheduled_message,
         # Immich events
         "assets_added": tc.track_assets_added,
         "assets_removed": tc.track_assets_removed,