docs(app): add UAT_PLAYBOOK.md section 4.20 for STOP/HELP consent handler

Adds 12 test cases covering:
- STOP/START/HELP flows and their auto-reply verification
- Alias keywords (STOPALL, UNSUBSCRIBE, CANCEL, END, QUIT / UNSTOP, YES, SUBSCRIBE, INFO)
- Idempotency for double STOP and double START
- Case-insensitivity and whitespace trimming
- Non-keyword message rejection
- Consent event audit log verification

Refs: GRO-1205, GRO-1469, PR #426

Co-Authored-By: Paperclip <noreply@paperclip.ing>

UAT_PLAYBOOK.md

@@ -235,6 +235,23 @@ GroomBook is an open-source, self-hostable pet grooming business management & CR
 | TC-APP-4.20.5 | Unread indicator | 1. Client sends a new message | Thread marked unread until staff views it |
 | TC-APP-4.20.6 | Cross-tenant isolation | 1. Staff from Business A attempts to read Business B conversations | 403 or empty response returned |
 
+
+### 4.21 SMS Consent (STOP/HELP Keyword Handler)
+
+| # | Scenario | Steps | Expected |
+|---|----------|-------|----------|
+| TC-APP-4.21.1 | STOP → unsubscribe + auto-reply | 1. Send `STOP` (case-insensitive, with whitespace) from a subscribed client's phone number | Client is opted out (`smsOptIn=false`, `smsOptOutDate` set), event is logged, user receives auto-reply: "You have been unsubscribed and will no longer receive messages. Reply START to resubscribe." |
+| TC-APP-4.21.2 | START → resubscribe + auto-reply | 1. Send `START` (case-insensitive) from an opted-out client's phone number | Client is opted back in (`smsOptIn=true`, `smsConsentDate` updated, `smsOptOutDate` cleared), event is logged, user receives auto-reply: "You have been resubscribed to messages. Reply STOP to unsubscribe. Msg & data rates may apply." |
+| TC-APP-4.21.3 | HELP → no opt-in change + default reply | 1. Send `HELP` (case-insensitive) from any client's phone number | No change to opt-in state, no database update, event is logged, user receives auto-reply: "Reply STOP to unsubscribe or START to resubscribe. For help, contact your groomer directly." |
+| TC-APP-4.21.4 | STOPALL / UNSUBSCRIBE / CANCEL / END / QUIT → opt-out | 1. Send each alias from a subscribed client's phone | Same behaviour as STOP: opt-out applied, correct reply sent |
+| TC-APP-4.21.5 | UNSTOP / YES / SUBSCRIBE → opt-in | 1. Send each alias from an opted-out client's phone | Same behaviour as START: opt-in applied, correct reply sent |
+| TC-APP-4.21.6 | INFO → help reply | 1. Send `INFO` from any client's phone | Same behaviour as HELP: no state change, help reply returned |
+| TC-APP-4.21.7 | Double STOP (idempotency) | 1. Send `STOP` from an already-opted-out client | Event is logged, no update call made, idempotent — no duplicate update |
+| TC-APP-4.21.8 | Double START (idempotency) | 1. Send `START` from an already-subscribed client | Event is logged, no update call made, idempotent — no duplicate update |
+| TC-APP-4.21.9 | Case insensitivity | 1. Send `stop`, `Stop`, `sToP`, ` stop ` from subscribed client | All variants are detected and handled as opt-out |
+| TC-APP-4.21.10 | Whitespace trimming | 1. Send `  START  ` or `\tSTOP\n` | Keywords are trimmed before matching |
+| TC-APP-4.21.11 | Non-keyword messages ignored | 1. Send `STOP IT`, `help me`, `hello` | Returns null from `detectKeyword`, no consent event inserted, no reply sent |
+| TC-APP-4.21.12 | Consent event audit log | 1. After any keyword, query `messageConsentEvents` table | Record exists with correct `clientId`, `businessId`, `kind`, and `source: "sms_keyword"` |
 ## 5. Pass/Fail Criteria
 
 **Pass:** All test cases execute without errors. Expected results match actual results. No regressions are observed. All functionality works as documented.