Verify mailer credentials against ProcessWeb API before showing E-Mail-Dienst as healthy

The Admin status pill previously turned green whenever the mailer was
enabled in config, without proving the account/token actually work.
Add IComService.VerifyCredentialsAsync (GET /mailer/fc?fn=verify, also
passing serverid when configured so the API confirms this server
belongs to the account) and wire it into a new "email" system-status
probe, so the pill only goes green once the Mailer API itself confirms
the credentials.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Fuchs/Docs/Concepts/admin-system-status.md

@@ -1,6 +1,6 @@
 ---
 status: Active
-lastUpdated: 2026-09-09
+lastUpdated: 2026-09-15
 applyTo:
   - "Fuchs/Controllers/IntranetController.Admin.cs"
   - "Fuchs/Services/SystemStatusService.cs"
@@ -56,15 +56,24 @@ click "Administration"  → $ocms.init('admin')
   **never the password**), email (mailer enabled/base-url/account/server-id/token-present +
   OverrideRecipient), blob (enabled/configured/containers), MFR (host/creds-present/sync),
   Key Vault (vault-uri/app-prefix/managed-secret-count/client-registered).
-- Probes (`database`, `keyvault`, `blob`, `mfr`) each return a `SystemProbeResult`
+- Probes (`database`, `email`, `keyvault`, `blob`, `mfr`) each return a `SystemProbeResult`
   (`status` = `ok`/`error`/`disabled`/`unconfigured`, `ok`, `message`, `detail`, `durationMs`,
   optional `metrics`). They never throw — failures are captured in the result. `database` runs
-  `SELECT @@SERVERNAME, DB_NAME(), SUSER_SNAME()`; `keyvault` reads the first managed secret via
-  the DI `SecretClient`; `blob` calls `IBlobStorageService.CheckConnectivityAsync` (account-info
-  request **plus a per-container blob count** for the invoice/reminder containers, surfaced as
-  `metrics` and totalled in the message); `mfr` calls `IMfrClientFactory.Create().GetEntities()`.
-  The generic `metrics` (label/value pairs) is how a probe reports extra facts for display — the
-  blob probe uses it for the file counts.
+  `SELECT @@SERVERNAME, DB_NAME(), SUSER_SNAME()`; `email` calls
+  `IComService.VerifyCredentialsAsync` (GET `{BaseUrl}/mailer/fc?fn=verify` with the configured
+  Basic-auth account/token — see the Mailer API contract) and only turns `ok` when the Mailer API
+  itself confirms the credentials (`ok:true`); when `ProcessWebComSettings.ServerId` is configured
+  it is sent along as `&serverid=...` so the API also confirms this server belongs to the account
+  (an account/token that is valid but for a server the account doesn't own comes back `ok:false`
+  with the mismatched `serverid` echoed, surfaced in `detail`); a configured-but-wrong token
+  surfaces as `error`, never as a false-positive `ok` — the `mailerEnabled`/`tokenConfigured` flags
+  in `GetInfo()` describe configuration only and were never proof the credentials actually work;
+  `keyvault` reads
+  the first managed secret via the DI `SecretClient`; `blob` calls
+  `IBlobStorageService.CheckConnectivityAsync` (account-info request **plus a per-container blob
+  count** for the invoice/reminder containers, surfaced as `metrics` and totalled in the message);
+  `mfr` calls `IMfrClientFactory.Create().GetEntities()`. The generic `metrics` (label/value pairs)
+  is how a probe reports extra facts for display — the blob probe uses it for the file counts.
 - `SendTestEmailAsync(to, subject, body)` HTML-encodes the body and sends via `IComService`, so
   the `Fuchs:Email:OverrideRecipient` redirect applies exactly as for any other mail; the result
   reports the requested recipient and, when active, the override target.