feat(auth): throttle login, register, password reset and Subsonic auth failures (M462 #4976)

Every password-shaped check was mounted bare, so guessing was limited only
by bcrypt cost. A shared in-memory AttemptLimiter now sits in front of them:

- login: 10 failures per account and 50 per address per 15 min, checked
  before the user lookup and bcrypt; 429 with Retry-After. A success clears
  the account's count but not the address's.
- unknown usernames run a dummy bcrypt compare, so timing no longer says
  which accounts exist.
- register: 10 per address per hour; forgot-password: 5 per address and 3
  per email per hour (applied whether or not the email matches); reset: 20
  failed tokens per address per 15 min.
- Subsonic /rest: same limits as login, counting only wrong credentials,
  since clients authenticate on every request.

Web login, register, reset and forgot-password screens say how long to
wait; web and Android carry copy for the rate_limited code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/routes/reset-password/[token]/+page.svelte

@@ -4,6 +4,7 @@
   import { pageTitle } from '$lib/branding';
   import { resetPassword } from '$lib/auth/store.svelte';
   import { errCode } from '$lib/api/errors';
+  import { rateLimitMessage } from '$lib/api/client';
 
   let newPassword = $state('');
   let confirmPassword = $state('');
@@ -29,7 +30,10 @@
       await goto('/login?reset=ok', { replaceState: true });
     } catch (e: unknown) {
       const code = errCode(e);
-      if (code === 'invalid_token') {
+      const throttled = rateLimitMessage(e);
+      if (throttled) {
+        error = throttled;
+      } else if (code === 'invalid_token') {
         error = 'This reset link is invalid, expired, or already used. Request a new one.';
       } else if (code === 'password_too_short') {
         error = 'Password must be at least 8 characters.';