Add SMS consent logging for Twilio compliance

Log phone, IP, and user agent to sms_consents table before sending
verification SMS. Add disclosure text to login form.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/login.html

@@ -18,6 +18,9 @@
       <label>Email or phone</label>
       <input type="text" name="identifier" placeholder="you@example.com or (555) 123-4567" required autofocus>
     </div>
+    <p style="font-family:'DM Mono',monospace;font-size:0.7rem;color:#777;margin-top:12px;line-height:1.5;">
+      By entering a phone number, you consent to receive a one-time verification code via SMS. Msg &amp; data rates may apply. No marketing messages will be sent.
+    </p>
     <button class="btn-submit" type="submit">Send code &#8599;</button>
   </form>
   {{else}}