Show job offer posting date (anzeige_datum) alongside ingestion date

- Add anzeige_datum DATE column to jobangebote (with ALTER TABLE migration
  for pre-existing tables) — the original posting date supplied by the
  third party
- Accept anzeige_datum in POST /api/v1/joboffers (create + upsert) and
  document it in the OpenAPI spec
- Render both on /jobangebote: "Anzeige: <anzeige_datum>" and
  "Einspielung: <created_at>"

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

server.js

@@ -619,6 +619,7 @@ function initializeDatabase() {
               beschreibung TEXT,
               quelle_url TEXT,
               art TEXT,
+              anzeige_datum DATE,
               status TEXT NOT NULL DEFAULT 'offen',
               verknuepfte_bewerbung_id INTEGER,
               created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
@@ -627,6 +628,8 @@ function initializeDatabase() {
               FOREIGN KEY (verknuepfte_bewerbung_id) REFERENCES bewerbungen(id) ON DELETE SET NULL
             )
           `);
+          // Migration: add anzeige_datum to pre-existing jobangebote tables.
+          db.run('ALTER TABLE jobangebote ADD COLUMN anzeige_datum DATE', () => {});
           // Remember the last recipient address per application (prefill).
           db.run('ALTER TABLE bewerbungen ADD COLUMN email_empfaenger TEXT', () => {});