build(functions): bump runtime Node 20 → 22 (Node 20 decommissioned 2026-10-30)

Node 20 was deprecated 2026-04-30 and deploys are blocked after 2026-10-30. nodejs22 is GA
for BOTH gens (deprecation 2027-04-30) — required because the runtime is codebase-wide and
onUserDelete stays gen1. nodejs24 was checked per plan and rejected: 2nd-gen only.

- functions/package.json engines.node 20 → 22 (the deploy-facing change; dist unaffected)
- .github/workflows/backend-ci.yml: both setup-node pins 20 → 22 so CI matches prod

Verified pre-deploy: tsc clean, 70 tests green, emulator discovery loads all 35 / 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/backend-ci.yml

@@ -22,7 +22,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: "20"
+          node-version: "22"
           cache: npm
           cache-dependency-path: functions/package-lock.json
 
@@ -51,7 +51,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: "20"
+          node-version: "22"
           cache: npm
           cache-dependency-path: firestore-tests/package-lock.json
 

functions/package.json

@@ -3,7 +3,7 @@
   "version": "1.0.0",
   "private": true,
   "engines": {
-    "node": "20"
+    "node": "22"
   },
   "main": "dist/index.js",
   "scripts": {