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/providers/+page.svelte

@@ -166,6 +166,13 @@
 				<input id="prv-token" bind:value={form.api_token} type="password" class="w-full px-3 py-2 border border-[var(--color-border)] rounded-md text-sm bg-[var(--color-background)]" />
 				<p class="text-xs text-[var(--color-muted-foreground)] mt-1">{t('providers.apiTokenHint')}</p>
 			</div>
+			{#if editing}
+			<div class="bg-[var(--color-muted)] rounded-md p-3">
+				<label class="block text-sm font-medium mb-1">{t('providers.webhookUrl')}</label>
+				<code class="text-xs select-all break-all">/api/webhooks/gitea/{editing}</code>
+				<p class="text-xs text-[var(--color-muted-foreground)] mt-1">{t('providers.webhookUrlHint')}</p>
+			</div>
+			{/if}
 			{/if}
 			<button type="submit" disabled={submitting}
 				class="px-4 py-2 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90 disabled:opacity-50">
@@ -196,6 +203,9 @@
 							{#if provider.config?.url}
 								<a href={provider.config.url} target="_blank" rel="noopener" class="text-xs text-[var(--color-muted-foreground)] font-mono hover:text-[var(--color-primary)] hover:underline">{provider.config.url}</a>
 							{/if}
+							{#if provider.type === 'gitea'}
+								<p class="text-xs text-[var(--color-muted-foreground)] font-mono mt-0.5">{t('providers.webhookUrl')}: <span class="select-all">/api/webhooks/gitea/{provider.id}</span></p>
+							{/if}
 						</div>
 					</div>
 					<div class="flex items-center gap-1">