feat(web/m7-cover-sources): cover-providers API client + query

Adds CoverProvider type + capability union, getCoverProviders /
updateCoverProvider / testCoverProvider against the three new admin
endpoints, plus createCoverProvidersQuery (60s staleTime; refetches
on focus + after mutations rather than polling — settings rarely
change in operator time).

New api.patch helper added to client.ts (the existing helpers were
get/post/put/del; PATCH was missing).

New qk.coverProviders() key follows the existing tuple pattern.
api_key_set: bool replaces echoing the credential. UpdateCoverProviderPatch
uses optional fields so the operator can change just enabled, just
key, or both atomically.

Vitest covers GET path, PATCH path with body, version_bumped
response, POST /test path with empty body, ok=false error response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/lib/api/client.ts

@@ -56,6 +56,8 @@ export const api = {
     apiFetch(path, { method: 'POST', body: JSON.stringify(body) }) as Promise<T>,
   put: <T>(path: string, body: unknown): Promise<T> =>
     apiFetch(path, { method: 'PUT', body: JSON.stringify(body) }) as Promise<T>,
+  patch: <T>(path: string, body: unknown): Promise<T> =>
+    apiFetch(path, { method: 'PATCH', body: JSON.stringify(body) }) as Promise<T>,
   del: (path: string): Promise<null> =>
     apiFetch(path, { method: 'DELETE' }) as Promise<null>
 };