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/services/sample_context.py

@@ -91,4 +91,50 @@ _SAMPLE_CONTEXT = {
     "photo_count": 30,
     "video_count": 5,
     "owner": "Alice",
+    # Gitea variables (for gitea provider templates)
+    "sender": "alexei",
+    "sender_name": "Alexei",
+    "sender_avatar": "",
+    "repo_name": "my-project",
+    "repo_full_name": "alexei/my-project",
+    "repo_url": "https://gitea.example.com/alexei/my-project",
+    "repo_description": "Example project",
+    "branch": "main",
+    "commits": [{"id": "abc1234567", "short_id": "abc1234", "message": "fix: example commit", "url": "", "author": "Alexei"}],
+    "commit_count": 1,
+    "compare_url": "https://gitea.example.com/alexei/my-project/compare/abc...def",
+    "issue_number": 42,
+    "issue_title": "Example issue",
+    "issue_url": "https://gitea.example.com/alexei/my-project/issues/42",
+    "issue_state": "open",
+    "issue_body": "",
+    "issue_labels": ["bug"],
+    "pr_number": 17,
+    "pr_title": "Add feature",
+    "pr_url": "https://gitea.example.com/alexei/my-project/pulls/17",
+    "pr_state": "open",
+    "pr_body": "",
+    "pr_merged": False,
+    "pr_base": "main",
+    "pr_head": "feature/example",
+    "pr_labels": [],
+    "comment_body": "Looks good!",
+    "comment_url": "",
+    "comment_author": "alexei",
+    "release_tag": "v1.0.0",
+    "release_name": "Version 1.0.0",
+    "release_url": "https://gitea.example.com/alexei/my-project/releases/tag/v1.0.0",
+    "release_body": "Initial release",
+    "release_draft": False,
+    "release_prerelease": False,
+    # Scheduler variables (for scheduler provider templates)
+    "schedule_name": "Daily Reminder",
+    "fire_count": 42,
+    "current_date": "22.03.2026",
+    "current_time": "09:00",
+    "current_datetime": "22.03.2026, 09:00 UTC",
+    "weekday": "Monday",
+    "custom_vars": {"team": "Engineering", "message": "Time for standup!"},
+    "team": "Engineering",
+    "message": "Time for standup!",
 }