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/registry.py

@@ -4,8 +4,11 @@ from __future__ import annotations
 
 # Map commands to rate limit categories
 _RATE_CATEGORY: dict[str, str] = {
+    # Immich
     "search": "search", "find": "search", "person": "search",
     "place": "search", "favorites": "search", "people": "search",
+    # Gitea (API calls share a category)
+    "repos": "api", "issues": "api", "prs": "api", "commits": "api",
 }
 
 
@@ -17,5 +20,5 @@ DEFAULT_COMMANDS_CONFIG = {
     "enabled": ["help", "status", "albums", "events", "latest", "random", "favorites", "summary", "memory"],
     "response_mode": "media",
     "default_count": 5,
-    "rate_limits": {"search": 30, "default": 10},
+    "rate_limits": {"search": 30, "api": 15, "default": 10},
 }