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

README.md

@@ -31,12 +31,15 @@ POST /submit
 Content-Type: application/json
 
 {
-  "email": "user@example.com",
-  "form_name": "newsletter",  # optional, default: "default"
-  "source": "homepage"        # optional
+  "email": "user@example.com",  # optional if message present
+  "form_name": "newsletter",    # optional, default: "default"
+  "source": "homepage",         # optional
+  "message": "free-text..."     # optional, max 10k chars; required if email absent
 }
 ```
 
+Either `email` or `message` is required. Message-only submissions (e.g. `form_name: "review_intercept"` from the iOS negative-review intercept) store an empty email.
+
 ### List Submissions (protected)
 ```bash
 GET /emails?form_name=newsletter&limit=100&offset=0