refactor: provider-agnostic bot command system + Gitea commands

Refactored the monolithic command handler (707 lines) into a pluggable
provider-handler architecture:

- Abstract ProviderCommandHandler interface (base.py)
- Handler dispatch registry routes commands by provider type
- Extracted all Immich logic into ImmichCommandHandler
- New GiteaCommandHandler with /status, /repos, /issues, /prs, /commits
- Multi-provider routing: groups context by provider type, finds handler
- handler.py reduced to ~280 line thin orchestrator

Gitea commands:
- Extended GiteaClient with get_repo_issues, get_repo_pulls, get_repo_commits
- 30 Jinja2 command templates (15 EN + 15 RU)
- Gitea capabilities updated with 6 commands + 15 command_slots
- Default command config + command template config seeded on startup
- Rate limiting: Gitea API commands share "api" category (15s cooldown)

Also:
- Command configs API accepts "gitea" provider type
- System command configs (user_id=0) visible to all users
- Webhook URL shown on Gitea provider card and edit form
- Scan interval hidden for webhook-based providers

packages/server/src/notify_bridge_server/commands/dispatch.py

@@ -0,0 +1,40 @@
+"""Provider command handler registry and routing."""
+
+from __future__ import annotations
+
+import logging
+
+from .base import ProviderCommandHandler
+
+_LOGGER = logging.getLogger(__name__)
+
+_REGISTRY: dict[str, ProviderCommandHandler] = {}
+
+
+def register_handler(handler: ProviderCommandHandler) -> None:
+    """Register a provider command handler."""
+    _REGISTRY[handler.provider_type] = handler
+    _LOGGER.debug("Registered command handler for provider type: %s", handler.provider_type)
+
+
+def get_handler(provider_type: str) -> ProviderCommandHandler | None:
+    """Get the command handler for a provider type."""
+    return _REGISTRY.get(provider_type)
+
+
+def get_all_handlers() -> dict[str, ProviderCommandHandler]:
+    """Get all registered handlers."""
+    return dict(_REGISTRY)
+
+
+def _auto_register() -> None:
+    """Auto-register all built-in handlers."""
+    from .immich_handler import ImmichCommandHandler
+    from .gitea_handler import GiteaCommandHandler
+
+    register_handler(ImmichCommandHandler())
+    register_handler(GiteaCommandHandler())
+
+
+# Auto-register on import
+_auto_register()