AI: status, gating and quota plumbing (`:feature:ai` scaffold)

[Adron]

Part of #3 — AI Writing Assistance is entirely missing from the Android app

Foundation for every other AI sub-issue.

• New self-contained :feature:ai module (own Retrofit AiApi built from the shared authed
  Retrofit, own DTOs) matching the per-feature module convention used elsewhere in this repo.
• GET /api/ai/status → an AiAvailability domain model: provider configured? subscriber?
  remaining daily quota?
• A single AiGate observable the feature UIs consume, so composer/list/document surfaces can
  hide their AI affordances entirely when AI is unavailable (unconfigured provider or free
  account) rather than showing a control that errors.
• Central mapping for the quota-exhausted response to a user-readable "daily AI limit reached"
  state.
• Capture the real /api/ai/status body before writing the DTOs — see the repo's API-envelope
  conventions (data wrapping, explicit nulls, coerceInputValues = true).

Tests: MockWebServer for status parse (configured / unconfigured / non-subscriber);
ViewModel test that the gate flips the UI off.

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Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.

[Adron]

Implemented in #92 (branch issue/4-ai-scaffold). Closing here — review and any follow-up happens on the PR.

Two things worth recording against the epic:

The question in this issue is answered: /api/ai/status does expose quota. Confirmed live with a subscriber token: { subscriber, providers:["anthropic"], defaultModels{anthropic,openai,gemini}, quota:{ usedToday, dailyLimit: 50, remaining } }.

There is a human-written API reference the parity analysis missed: https://interlinedlist.com/help/api/ai-integration documents the full request/response shapes, error codes and limits for all three AI endpoints — everything the auto-generated OpenAPI spec omits (it types them as a bare object). Sibling issues #5–#9 should start there rather than from the spec.