refactor(server): replace node-fetch with native fetch + undici, fix photo integrations

Replace node-fetch v2 with Node 22's built-in fetch API across the entire server.
Add undici as an explicit dependency to provide the dispatcher API needed for
DNS pinning (SSRF rebinding prevention) in ssrfGuard.ts. All seven service files
that used a plain `import fetch from 'node-fetch'` are updated to use the global.
The ssrfGuard safeFetch/createPinnedAgent is rewritten as createPinnedDispatcher
using an undici Agent, with correct handling of the `all: true` lookup callback
required by Node 18+. The collabService dynamic require() and notifications agent
option are updated to use the dispatcher pattern. Test mocks are migrated from
vi.mock('node-fetch') to vi.stubGlobal('fetch'), and streaming test fixtures are
updated to use Web ReadableStream instead of Node Readable.

Fix several bugs in the Synology and Immich photo integrations:
- pipeAsset: guard against setting headers after stream has already started
- _getSynologySession: clear stale SID and re-login when decrypt_api_key returns null
  instead of propagating success(null) downstream
- _requestSynologyApi: return retrySession error (not stale session) on retry failure;
  also retry on error codes 106 (timeout) and 107 (duplicate login), not only 119
- searchSynologyPhotos: fix incorrect total field type (Synology list_item returns no
  total); hasMore correctly uses allItems.length === limit
- _splitPackedSynologyId: validate cache_key format before use; callers return 400
- getImmichCredentials / _getSynologyCredentials: treat null from decrypt_api_key as
  a missing-credentials condition rather than casting null to string
- Synology size param: enforce allowlist ['sm', 'm', 'xl'] per API documentation

server/src/utils/ssrfGuard.ts

@@ -1,6 +1,5 @@
 import dns from 'node:dns/promises';
-import http from 'node:http';
-import https from 'node:https';
+import { Agent } from 'undici';
 
 const ALLOW_INTERNAL_NETWORK = process.env.ALLOW_INTERNAL_NETWORK === 'true';
 
@@ -106,22 +105,46 @@ export async function checkSsrf(rawUrl: string, bypassInternalIpAllowed: boolean
 }
 
 /**
- * Returns an http/https Agent whose `lookup` function is pinned to the
- * already-validated IP. This prevents DNS rebinding (TOCTOU) by ensuring
- * the outbound connection goes to the IP we checked, not a re-resolved one.
+ * Thrown by safeFetch() when the URL is blocked by the SSRF guard.
  */
-export function createPinnedAgent(resolvedIp: string, protocol: string): http.Agent | https.Agent {
-  const options = {
-    lookup: (_hostname: string, opts: Record<string, unknown>, callback: Function) => {
-      // Determine address family from IP format
-      const family = resolvedIp.includes(':') ? 6 : 4;
-      // Node.js 18+ may call lookup with `all: true`, expecting an array of address objects
-      if (opts && opts.all) {
-        callback(null, [{ address: resolvedIp, family }]);
-      } else {
-        callback(null, resolvedIp, family);
-      }
-    },
-  };
-  return protocol === 'https:' ? new https.Agent(options) : new http.Agent(options);
+export class SsrfBlockedError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'SsrfBlockedError';
+  }
+}
+
+/**
+ * SSRF-safe fetch wrapper. Validates the URL with checkSsrf(), then makes
+ * the request using a DNS-pinned dispatcher so the resolved IP cannot change
+ * between the check and the actual connection (DNS rebinding prevention).
+ */
+export async function safeFetch(url: string, init?: RequestInit): Promise<Response> {
+  const ssrf = await checkSsrf(url);
+  if (!ssrf.allowed) {
+    throw new SsrfBlockedError(ssrf.error ?? 'Request blocked by SSRF guard');
+  }
+  const dispatcher = createPinnedDispatcher(ssrf.resolvedIp!);
+  return fetch(url, { ...init, dispatcher } as any);
+}
+
+/**
+ * Returns an undici Agent whose connect.lookup is pinned to the already-validated
+ * IP. This prevents DNS rebinding (TOCTOU) by ensuring the outbound connection
+ * goes to the IP we checked, not a re-resolved one.
+ */
+export function createPinnedDispatcher(resolvedIp: string): Agent {
+  return new Agent({
+    connect: {
+      lookup: (_hostname: string, opts: Record<string, unknown>, callback: Function) => {
+        const family = resolvedIp.includes(':') ? 6 : 4;
+        // Node.js 18+ may call lookup with `all: true`, expecting an array of address objects
+        if (opts?.all) {
+          callback(null, [{ address: resolvedIp, family }]);
+        } else {
+          callback(null, resolvedIp, family);
+        }
+      },
+    },
+  });
 }