feat(job-search): read providers from DB and suppress link when none enabled

JobTokenService.CreateTokenAsync queries cvSearch.JobProviders for any
enabled row; returns null (no token created) when the table is empty or
all providers are disabled. TriggerStartAsync snapshots enabled providers
from DB at session-start time, preserving the existing snapshot contract.

CvMatcherController guards link-building on a non-null TokenId so the
"Start a job search" CTA is omitted from match emails when no providers
are configured.

JobSearchSettings.Providers list removed — provider config now lives
exclusively in the DB. CvSearchJobTask.GetProviders falls back to an
empty list with a warning (snapshot should always be populated from DB).

Closes #35

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

Apis/cv-matcher-api/Services/Contracts/IJobTokenService.cs

@@ -13,8 +13,11 @@ public interface IJobTokenService
     /// <param name="email">Email address of the user who will receive the results.</param>
     /// <param name="language">Preferred language for result emails (e.g. <c>"en"</c>, <c>"ro"</c>).</param>
     /// <param name="ct">Cancellation token.</param>
-    /// <returns>The generated token ID, to be embedded in the one-click job search link.</returns>
-    Task<string> CreateTokenAsync(string cvDocumentId, string email, string language, CancellationToken ct);
+    /// <returns>
+    /// The generated token ID to embed in the one-click job search link,
+    /// or <c>null</c> when no job providers are currently enabled (link should be suppressed).
+    /// </returns>
+    Task<string?> CreateTokenAsync(string cvDocumentId, string email, string language, CancellationToken ct);
 
     /// <summary>
     /// Validates the token and, if valid, marks it as used and creates a <c>Pending</c> job search session.