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>

Jobs/cv-search-job/Tasks/CvSearchJobTask.cs

@@ -204,20 +204,27 @@ public sealed class CvSearchJobTask : IJobTask
 
     /// <summary>
     /// Deserialises the provider configuration snapshot stored on the session.
-    /// Falls back to the current live config when the snapshot is absent or unparseable.
+    /// Providers are always snapshotted from the DB at session-creation time, so the snapshot
+    /// should always be present. Returns an empty list (with a warning) when it is missing or corrupt.
     /// </summary>
     private List<JobProviderConfig> GetProviders(string? providerConfigJson)
     {
-        if (string.IsNullOrWhiteSpace(providerConfigJson)) return _settings.Providers.Where(p => p.Enabled).ToList();
+        if (string.IsNullOrWhiteSpace(providerConfigJson))
+        {
+            _logger.LogWarning("Session has no provider config snapshot — returning empty provider list");
+            return [];
+        }
+
         try
         {
             return JsonSerializer.Deserialize<List<JobProviderConfig>>(providerConfigJson,
                        new JsonSerializerOptions(JsonSerializerDefaults.Web))
-                   ?? _settings.Providers.Where(p => p.Enabled).ToList();
+                   ?? [];
         }
-        catch
+        catch (Exception ex)
         {
-            return _settings.Providers.Where(p => p.Enabled).ToList();
+            _logger.LogWarning(ex, "Failed to deserialise provider config snapshot — returning empty provider list");
+            return [];
         }
     }