[Feature] Configurable upload URL — send asset uploads to a different host than browsing

[Feature] Configurable upload URL — send asset uploads to a different host than browsing Problem Immich uploads each asset as a single, non-resumable POST /api/assets. When the server is reached through a proxy or tunnel that caps request-body size, any file above that cap fails with 413 and there is no client-side workaround. The most common case is Cloudflare's 100 MB body limit on the Free/Pro plans (and via Cloudflare Tunnel), which blocks large videos. This is already reported and discussed: • #25183 — Uploads fail behind Cloudflare Free due to the 100 MB limit • #1674 — Upload large files in chunks (the "proper" long-term fix; still open) Chunked/resumable upload (#1674) would solve the root cause but is a large piece of work. This proposal is a small, orthogonal improvement that helps today without touching the transfer protocol. Why the existing endpoint switching doesn't cover it The app's Automatic URL Switching swaps the *single* s…