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>

internal/api/auth_register.go

@@ -55,6 +55,15 @@ type registerReq struct {
 //   - 409 username_taken (PG unique violation on users.username)
 //   - 500 server_error otherwise
 func (h *handlers) handleRegister(w http.ResponseWriter, r *http.Request) {
+	// Every attempt counts, not only failures: a successful registration is
+	// the thing being sprayed when the mode is open.
+	addr := auth.ClientIP(r, h.netSettings.Hops())
+	if blocked, wait := h.registerLimit.Blocked(addr); blocked {
+		writeRateLimited(w, wait)
+		return
+	}
+	h.registerLimit.Record(addr)
+
 	var req registerReq
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeErr(w, apierror.BadRequest("invalid_body", "invalid JSON body"))
@@ -175,7 +184,7 @@ func (h *handlers) handleRegister(w http.ResponseWriter, r *http.Request) {
 		UserID:    user.ID,
 		TokenHash: auth.HashSessionToken(sessionToken),
 		UserAgent: r.UserAgent(),
-		Ip:        auth.ClientIP(r, h.netSettings.Hops()),
+		Ip:        addr,
 	}); err != nil {
 		h.logger.Error("register: insert session failed", "err", err)
 		writeErr(w, apierror.Internal(err))