fix: replace JWT tokens in URL query params with short-lived ephemeral tokens

Addresses CWE-598: long-lived JWTs were exposed in WebSocket URLs, file
download links, and Immich asset proxy URLs, leaking into server logs,
browser history, and Referer headers.

- Add ephemeralTokens service: in-memory single-use tokens with per-purpose
  TTLs (ws=30s, download/immich=60s), max 10k entries, periodic cleanup
- Add POST /api/auth/ws-token and POST /api/auth/resource-token endpoints
- WebSocket auth now consumes an ephemeral token instead of verifying the JWT
  directly from the URL; client fetches a fresh token before each connect
- File download ?token= query param now accepts ephemeral tokens; Bearer
  header path continues to accept JWTs for programmatic access
- Immich asset proxy replaces authFromQuery JWT injection with ephemeral token
  consumption
- Client: new getAuthUrl() utility, AuthedImg/ImmichImg components, and async
  onClick handlers replace the synchronous authUrl() pattern throughout
  FileManager, PlaceInspector, and MemoriesPanel
- Add OIDC_DISCOVERY_URL env var and oidc_discovery_url DB setting to allow
  overriding the auto-constructed discovery endpoint (required for Authentik
  and similar providers); exposed in the admin UI and .env.example

client/src/api/authUrl.ts

@@ -0,0 +1,19 @@
+export async function getAuthUrl(url: string, purpose: 'download' | 'immich'): Promise<string> {
+  const jwt = localStorage.getItem('auth_token')
+  if (!jwt || !url) return url
+  try {
+    const resp = await fetch('/api/auth/resource-token', {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        'Authorization': `Bearer ${jwt}`,
+      },
+      body: JSON.stringify({ purpose }),
+    })
+    if (!resp.ok) return url
+    const { token } = await resp.json()
+    return `${url}${url.includes('?') ? '&' : '?'}token=${token}`
+  } catch {
+    return url
+  }
+}