ADR-207: The DeepL character quota is shown on the test page 

Context 

DeepL bills by the character against an account quota. The Free API stops at its monthly limit; a Pro account keeps billing, often up to a cost limit the account owner set. DeepLTranslator::getUsage() has asked DeepL's /v2/usage endpoint for the characters used and the limit since the translator was written, and nothing called it. An administrator learned that the quota was spent when translations started failing with DeepL's status 456.

DeepL is not an LLM provider record. It is configured in the Extension Configuration (translators.deepl.*), so the provider list, the provider "Test connection" action and the analytics dashboard do not know it. The one backend surface that does is the test page of ADR-118: its translation card lists the registered translators, marks the ones without a credential and translates through DeepL on request.

Decision 

The quota is shown in the translation card of the test page, through a fourth AJAX route on SpecializedTestController. No new module, no new card.

• nrllm_test_deepl_quota calls deeplQuotaAction(), which asks the translation registry for deepl and returns used, limit, usedPercent and plan.
• The page asks it when it opens and again after a DeepL translation. The usage endpoint costs no characters, which is why this is the one specialized endpoint that runs without a button.
• CharacterQuotaReportingInterface declares getCharacterQuota(): CharacterQuota, and DeepLTranslator implements it. The controller depends on the interface, not the class: DeepLTranslator is final, and ADR-118 solved the same problem for the image services with ImageGeneratorInterface. It is not part of TranslatorInterface, because most translators have no character quota, and adding a method there would break every third-party translator. A translator registered as deepl that does not implement it gets a 501 with a plain message.
• plan names the endpoint that answered: free for api-free.deepl.com, pro for api.deepl.com, custom for a configured translators.deepl.baseUrl. It is read from the endpoint DeepLTranslator already chose from the key — a Free key ends in :fx — so the key is not read a second time to label it.
• A limit of 0 means DeepL reported none; the share used is then null rather than a division by zero.

Failures answer as the translation test does. No credential, or DeepL out of reach, is a 503 naming the Extension Configuration. A key DeepL refuses is a 502 saying the key was rejected. Anything else is a 500 with the detail in the log. None of the three carries the exception message, and that matters here: the message of a failed DeepL call includes the text DeepL sent back, and the key must not reach the page by that route either.

Consequences 

• An administrator sees how much of the DeepL quota is left before translations fail, and whether the configured key is a Free or a Pro key.
• Opening the test page sends one request to DeepL whenever a DeepL key is configured. It costs no characters and is admin-only (ADR-037).
• The action adds a credential-bearing enforcement point to SpecializedTestController; the inventory in ADR-169 section 7 counted 40 sites before it.
• The quota is shown only on the test page. A dashboard widget or a warning near the limit would need the quota stored or polled; neither is built.