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>

server.js

@@ -620,6 +620,7 @@ function initializeDatabase() {
               quelle_url TEXT,
               art TEXT,
               anzeige_datum DATE,
+              kontakt_email TEXT,
               status TEXT NOT NULL DEFAULT 'offen',
               verknuepfte_bewerbung_id INTEGER,
               created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
@@ -628,8 +629,9 @@ function initializeDatabase() {
               FOREIGN KEY (verknuepfte_bewerbung_id) REFERENCES bewerbungen(id) ON DELETE SET NULL
             )
           `);
-          // Migration: add anzeige_datum to pre-existing jobangebote tables.
+          // Migration: add columns to pre-existing jobangebote tables.
           db.run('ALTER TABLE jobangebote ADD COLUMN anzeige_datum DATE', () => {});
+          db.run('ALTER TABLE jobangebote ADD COLUMN kontakt_email TEXT', () => {});
           // Remember the last recipient address per application (prefill).
           db.run('ALTER TABLE bewerbungen ADD COLUMN email_empfaenger TEXT', () => {});