auth: replace UUID-based login with JWT credential verification

Login now requires login/email + password verified against DB via
/api/auth/login. Only approved registrations can access the app.
Signal endpoint accepts JWT Bearer tokens alongside legacy api_key auth.
Old UUID-only registration flow removed from frontend.

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

frontend/index.html

@@ -37,23 +37,32 @@
   <!-- Onboarding screen: shown on first visit (no UUID registered yet) -->
   <div id="screen-onboarding" class="screen" role="main" hidden>
 
-    <!-- View: name entry (existing onboarding) -->
+    <!-- View: login with credentials -->
     <div class="screen-content" id="view-login">
       <input
         type="text"
-        id="name-input"
+        id="login-input"
         class="name-input"
-        placeholder="Your name"
-        maxlength="100"
-        autocomplete="name"
+        placeholder="Логин или email"
+        maxlength="255"
+        autocomplete="username"
         autocorrect="off"
-        autocapitalize="words"
+        autocapitalize="none"
         spellcheck="false"
-        aria-label="Your name"
+        aria-label="Логин или email"
       >
-      <button type="button" id="btn-confirm" class="btn-confirm" disabled>
-        Confirm
+      <input
+        type="password"
+        id="login-password"
+        class="name-input"
+        placeholder="Пароль"
+        autocomplete="current-password"
+        aria-label="Пароль"
+      >
+      <button type="button" id="btn-login" class="btn-confirm" disabled>
+        Войти
       </button>
+      <div id="login-status" class="reg-status" hidden></div>
       <button type="button" id="btn-switch-to-register" class="btn-link">
         Зарегистрироваться
       </button>