Add optional message field to /submit for iOS review intercept

- /submit now requires email OR message (email-only calls unchanged)
- message trimmed + capped at 10k chars; stored via guarded ALTER TABLE migration
- /export CSV gains message column with proper quoting/escaping
- test suite (node --test): 9 tests covering compat, validation, truncation, CSV escaping

Ticket: T2 of review-flow-10x epic (chatai-ios#136)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014WPnLnXS4YRz9vVt1BEdeR

package.json

@@ -4,7 +4,8 @@
   "description": "Lightweight form capture microservice",
   "main": "index.js",
   "scripts": {
-    "start": "node index.js"
+    "start": "node index.js",
+    "test": "node --test"
   },
   "dependencies": {
     "sql.js": "^1.11.0",