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

frontend/src/lib/i18n/en.json

@@ -119,6 +119,8 @@
 		"webhookSecretRequired": "Webhook secret is required",
 		"apiToken": "API Token",
 		"apiTokenHint": "Optional. Needed for connection testing and repository listing.",
+		"webhookUrl": "Webhook URL",
+		"webhookUrlHint": "Set this as the Target URL in Gitea webhook settings (relative to your bridge host).",
 		"testAndSave": "Test & Save",
 		"saveWithoutTest": "Save without testing"
 	},

frontend/src/lib/i18n/ru.json

@@ -119,6 +119,8 @@
 		"webhookSecretRequired": "Секрет вебхука обязателен",
 		"apiToken": "API токен",
 		"apiTokenHint": "Необязательно. Нужен для проверки подключения и получения списка репозиториев.",
+		"webhookUrl": "URL вебхука",
+		"webhookUrlHint": "Укажите этот URL в настройках вебхука Gitea (относительно хоста bridge).",
 		"testAndSave": "Проверить и сохранить",
 		"saveWithoutTest": "Сохранить без проверки"
 	},