Switch to phone auth via Twilio SMS and add feature flag system

Replace email-based auth (Resend) with phone-based auth (Twilio SMS).
Add BBQ_FEATURES env var for toggling features at deploy time — when
auth is disabled, no login routes are registered and the app works
as before.

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

templates/event.html

@@ -49,7 +49,7 @@
         hx-on::after-request="if(event.detail.successful) this.reset()">
     <div class="form-row">
       <label>Your name</label>
-      <input type="text" name="name" placeholder="e.g. Sam" required>
+      <input type="text" name="name" placeholder="e.g. Sam" required value="{{if .User}}{{.User.Name}}{{end}}">
     </div>
     <div class="form-row">
       <label>Note (optional)</label>
@@ -70,7 +70,7 @@
         hx-on::after-request="if(event.detail.successful) this.reset()">
     <div class="form-row">
       <label>Your name</label>
-      <input type="text" name="name" placeholder="e.g. Sam" required>
+      <input type="text" name="name" placeholder="e.g. Sam" required value="{{if .User}}{{.User.Name}}{{end}}">
     </div>
     <div class="form-row">
       <label>Slot</label>