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/scheduler.py

@@ -40,7 +40,7 @@ async def _load_tracker_jobs() -> None:
     from sqlmodel import select
     from sqlmodel.ext.asyncio.session import AsyncSession
     from ..database.engine import get_engine
-    from ..database.models import NotificationTracker
+    from ..database.models import NotificationTracker, ServiceProvider as ServiceProviderModel
 
     engine = get_engine()
     scheduler = get_scheduler()
@@ -49,11 +49,35 @@ async def _load_tracker_jobs() -> None:
         result = await session.exec(select(NotificationTracker).where(NotificationTracker.enabled == True))
         trackers = result.all()
 
+        # Pre-load provider types for scheduler detection
+        provider_types: dict[int, str] = {}
+        for tracker in trackers:
+            if tracker.provider_id not in provider_types:
+                provider = await session.get(ServiceProviderModel, tracker.provider_id)
+                if provider:
+                    provider_types[tracker.provider_id] = provider.type
+
     for tracker in trackers:
         job_id = f"tracker_{tracker.id}"
         if scheduler.get_job(job_id):
             continue
 
+        ptype = provider_types.get(tracker.provider_id, "")
+        filters = tracker.filters or {}
+
+        # Scheduler providers can use cron triggers
+        if ptype == "scheduler" and filters.get("schedule_type") == "cron":
+            cron_expr = filters.get("cron_expression", "")
+            if cron_expr:
+                try:
+                    _add_cron_job(scheduler, job_id, tracker.id, cron_expr, tracker.name)
+                    continue
+                except Exception as e:
+                    _LOGGER.error(
+                        "Invalid cron expression for tracker %d (%s): %s — falling back to interval",
+                        tracker.id, tracker.name, e,
+                    )
+
         scheduler.add_job(
             _poll_tracker,
             "interval",
@@ -65,23 +89,55 @@ async def _load_tracker_jobs() -> None:
         _LOGGER.info("Scheduled tracker %d (%s) every %ds", tracker.id, tracker.name, tracker.scan_interval)
 
 
-async def schedule_tracker(tracker_id: int, interval: int) -> None:
+def _add_cron_job(
+    scheduler: AsyncIOScheduler,
+    job_id: str,
+    tracker_id: int,
+    cron_expression: str,
+    tracker_name: str,
+) -> None:
+    """Add a cron-triggered job for a scheduler-type tracker."""
+    from apscheduler.triggers.cron import CronTrigger
+    trigger = CronTrigger.from_crontab(cron_expression)
+    scheduler.add_job(
+        _poll_tracker,
+        trigger,
+        id=job_id,
+        args=[tracker_id],
+        replace_existing=True,
+    )
+    _LOGGER.info("Scheduled tracker %d (%s) with cron: %s", tracker_id, tracker_name, cron_expression)
+
+
+async def schedule_tracker(
+    tracker_id: int,
+    interval: int,
+    cron_expression: str | None = None,
+) -> None:
     """Add or update a scheduler job for a tracker."""
     scheduler = get_scheduler()
     job_id = f"tracker_{tracker_id}"
+
+    # Remove existing job first to allow trigger type changes
     if scheduler.get_job(job_id):
-        scheduler.reschedule_job(job_id, trigger="interval", seconds=interval)
-        _LOGGER.info("Rescheduled tracker %d every %ds", tracker_id, interval)
-    else:
-        scheduler.add_job(
-            _poll_tracker,
-            "interval",
-            seconds=interval,
-            id=job_id,
-            args=[tracker_id],
-            replace_existing=True,
-        )
-        _LOGGER.info("Scheduled tracker %d every %ds", tracker_id, interval)
+        scheduler.remove_job(job_id)
+
+    if cron_expression:
+        try:
+            _add_cron_job(scheduler, job_id, tracker_id, cron_expression, f"tracker-{tracker_id}")
+            return
+        except Exception as e:
+            _LOGGER.error("Invalid cron for tracker %d: %s — using interval", tracker_id, e)
+
+    scheduler.add_job(
+        _poll_tracker,
+        "interval",
+        seconds=interval,
+        id=job_id,
+        args=[tracker_id],
+        replace_existing=True,
+    )
+    _LOGGER.info("Scheduled tracker %d every %ds", tracker_id, interval)
 
 
 async def unschedule_tracker(tracker_id: int) -> None: