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-models/Responses/CreateJobSearchTokenResponse.cs

@@ -2,5 +2,9 @@ namespace CvMatcher.Models.Responses;
 
 public sealed class CreateJobSearchTokenResponse
 {
-    public string TokenId { get; set; } = string.Empty;
+    /// <summary>
+    /// The generated token ID, or <c>null</c> when no job providers are currently enabled.
+    /// Callers must check for null before building the job-search link.
+    /// </summary>
+    public string? TokenId { get; set; }
 }

Apis/cv-matcher-api-models/Settings/JobSearchSettings.cs

@@ -7,9 +7,12 @@ public sealed class JobSearchSettings
     public int TokenExpiryDays { get; set; } = 7;
     public int MinMatchScore { get; set; } = 15;
     public int MaxJobsToMatch { get; set; } = 15;
-    public List<JobProviderConfig> Providers { get; set; } = [];
 }
 
+/// <summary>
+/// Runtime DTO for a job provider. Populated from <c>cvSearch.JobProviders</c> at session-creation
+/// time and snapshotted to <c>JobSearchSessionEntity.ProviderConfigJson</c>.
+/// </summary>
 public sealed class JobProviderConfig
 {
     public string Name { get; set; } = string.Empty;