Add smart combined album asset redistribution + fix locale string

Core library:
- New combine_album_assets() in asset_utils.py: smart redistribution
  of unused quota when albums return fewer assets than their share.
  Two-pass algorithm: even split then redistribute remainder.
- 6 new tests (56 total passing).

Frontend:
- Fix "leave empty to keep current" not localized in server edit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/servers/+page.svelte

@@ -71,7 +71,7 @@
 				<input id="srv-url" bind:value={form.url} required placeholder={t('servers.urlPlaceholder')} class="w-full px-3 py-2 border border-[var(--color-border)] rounded-md text-sm bg-[var(--color-background)]" />
 			</div>
 			<div>
-				<label for="srv-key" class="block text-sm font-medium mb-1">{t('servers.apiKey')}{editing ? ' (leave empty to keep current)' : ''}</label>
+				<label for="srv-key" class="block text-sm font-medium mb-1">{editing ? t('servers.apiKeyKeep') : t('servers.apiKey')}</label>
 				<input id="srv-key" bind:value={form.api_key} type="password" required={!editing} class="w-full px-3 py-2 border border-[var(--color-border)] rounded-md text-sm bg-[var(--color-background)]" />
 			</div>
 			<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">