Add kontakt_email to job offers

- New kontakt_email TEXT column on jobangebote (+ ALTER TABLE migration)
- Accept kontakt_email in POST /api/v1/joboffers (create + upsert) and
  document it in the OpenAPI spec
- Render a mailto: link on the /jobangebote page and include it in the
  curl example

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

lib/openapi.js

@@ -785,6 +785,7 @@ function buildOpenApiSpec(baseUrl = '') {
             quelle_url: { type: 'string', nullable: true },
             art: { type: 'string', nullable: true },
             anzeige_datum: { type: 'string', format: 'date', nullable: true, description: 'Datum der eigentlichen Stellenanzeige (vom Drittanbieter übergeben)' },
+            kontakt_email: { type: 'string', format: 'email', nullable: true, description: 'Kontakt-E-Mail-Adresse der Stelle' },
             status: { type: 'string', enum: ['offen', 'uebernommen', 'abgelehnt'] },
             verknuepfte_bewerbung_id: { type: 'integer', nullable: true },
             created_at: { type: 'string', format: 'date-time' },
@@ -805,6 +806,7 @@ function buildOpenApiSpec(baseUrl = '') {
             quelle_url: { type: 'string' },
             art: { type: 'string', enum: ART_OPTIONS },
             anzeige_datum: { type: 'string', format: 'date', description: 'Datum der eigentlichen Stellenanzeige (ISO YYYY-MM-DD)' },
+            kontakt_email: { type: 'string', format: 'email', description: 'Kontakt-E-Mail-Adresse der Stelle' },
             status: { type: 'string', enum: ['offen', 'uebernommen', 'abgelehnt'], default: 'offen' },
           },
         },