Add Immich photo storage backend with demo page and connection settings

[Copilot]

Adds Immich as a photo storage backend with a configurable connection, an upload-through-middleware flow, and a gallery whose image bytes are fetched directly from Immich by the browser (never proxied through PRM).

Backend (server/routes.ts)

• New keys in app_settings: immich_enabled, immich_url, immich_api_key (reusing the existing key/value table — no schema changes).
• GET/POST /api/immich/settings — read/write config; API key never returned, only hasApiKey.
• POST /api/immich/test — auth + connectivity check via Immich /api/users/me.
• POST /api/immich/upload (multipart) — forwards to Immich /api/assets with x-api-key, then inserts into the photos table with prmLocation = "immich_demo:<assetId>" and location = <url>/api/assets/<assetId>/original.
• GET /api/immich/assets — proxies /api/search/metadata to list recent IMAGE assets.
• GET /api/immich/client-config — returns { enabled, url, apiKey } to authenticated clients so the browser can call Immich directly for image bytes.

Frontend

• pages/immich-settings.tsx at /settings/immich (under App Options): enable toggle, URL, API key, single Save and test button.

• pages/immich-demo.tsx at /immich-demo: drop-zone upload via /api/immich/upload; thumbnail grid where each tile fetches directly from Immich:

  const resp = await fetch(`${url}/api/assets/${assetId}/thumbnail?size=preview`, {
    headers: { "x-api-key": apiKey },
  });
  setSrc(URL.createObjectURL(await resp.blob()));

• Main sidebar entry for "Immich Demo" is conditional on immich_enabled. Settings sidebar, router, and page-title wired up.