Rate-limit the password change endpoint (#65)

The password change POST verifies the current password, making it
a password-based authentication endpoint that REPO_POLICIES.md
requires rate limiting on. Extract the login limiter's POST-only
per-IP pattern into a shared postRateLimit helper and apply the
same 5-per-minute limit to POST /user/{username}/password.

internal/middleware/export_test.go

@@ -32,3 +32,7 @@ func IsClientTLS(r *http.Request) bool {
 
 // LoginRateLimitConst exposes the loginRateLimit constant.
 const LoginRateLimitConst = loginRateLimit
+
+// PasswordChangeRateLimitConst exposes the
+// passwordChangeRateLimit constant.
+const PasswordChangeRateLimitConst = passwordChangeRateLimit