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/routes/notification-trackers/TrackerForm.svelte

@@ -45,6 +45,7 @@
 	}: Props = $props();
 
 	let isScheduler = $derived(providerType === 'scheduler');
+	let isWebhook = $derived(providerType === 'gitea');
 
 	// Custom variable management for scheduler
 	function addVariable() {
@@ -162,10 +163,12 @@
 			</fieldset>
 		{:else}
 		<div class="grid grid-cols-2 gap-3">
+			{#if !isWebhook}
 			<div>
 				<label for="trk-interval" class="block text-sm font-medium mb-1">{t('notificationTracker.scanInterval')}<Hint text={t('hints.scanInterval')} /></label>
 				<input id="trk-interval" type="number" bind:value={form.scan_interval} min="10" max="3600" class="w-full px-3 py-2 border border-[var(--color-border)] rounded-md text-sm bg-[var(--color-background)]" />
 			</div>
+			{/if}
 			<div>
 				<label for="trk-batch" class="block text-sm font-medium mb-1">{t('notificationTracker.batchDuration')}<Hint text={t('hints.batchDuration')} /></label>
 				<input id="trk-batch" type="number" bind:value={form.batch_duration} min="0" max="3600" class="w-full px-3 py-2 border border-[var(--color-border)] rounded-md text-sm bg-[var(--color-background)]" />