feat: support inline editing of custom provider endpoints, keys, model names, and model adding

[gofulljs]

Summary

Extends custom provider key management with inline editing capabilities.

Backend (server/src/routes/)

• keys.ts: PATCH /:id now supports updating baseUrl and apiKey fields (with SSRF check + duplicate URL detection). When a custom key label changes, linked models' display_name is auto-updated.
• fallback.ts: Custom models' platform name now uses api_keys.label instead of hardcoded "custom", so model pages and playground show meaningful names.

Frontend (client/src/pages/KeysPage.tsx)

• Endpoint & Key inline editing: Expanded custom key rows show Base URL and API Key inputs; save via button.
• Model name editing: Click a model name in expanded view to edit display_name inline.
• Add model: "+ Add model" button at bottom of expanded view with type selector (chat/embedding/image/audio).
• Platform label display: Model pages and playground show key label instead of "custom".

i18n

Added keys.save translation key to all 6 locale files.

Testing

• Server tsc passes
• Client tsc -b && vite build passes
• Docker build and deploy verified