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>

lib/api.js

@@ -536,6 +536,7 @@ function createExternalApi(deps) {
       }
       const quelle = sanitizeInput(b.quelle || 'drittanbieter');
       const externalId = b.external_id != null ? sanitizeInput(String(b.external_id)) : null;
+      const anzeigeDatum = sanitizeInput(b.anzeige_datum || '');
       const fields = [
         sanitizeInput(b.firma),
         sanitizeInput(b.stelle),
@@ -544,6 +545,7 @@ function createExternalApi(deps) {
         sanitizeInput(b.beschreibung || ''),
         sanitizeInput(b.quelle_url || ''),
         sanitizeInput(b.art || ''),
+        anzeigeDatum,
         sanitizeInput(b.status || 'offen'),
       ];
 
@@ -556,7 +558,7 @@ function createExternalApi(deps) {
         if (existing) {
           await dbRun(
             `UPDATE jobangebote SET firma = ?, stelle = ?, ort = ?, gehalt = ?,
-               beschreibung = ?, quelle_url = ?, art = ?, status = ?,
+               beschreibung = ?, quelle_url = ?, art = ?, anzeige_datum = ?, status = ?,
                updated_at = CURRENT_TIMESTAMP WHERE id = ?`,
             [...fields, existing.id]
           );
@@ -567,8 +569,8 @@ function createExternalApi(deps) {
 
       const result = await dbRun(
         `INSERT INTO jobangebote
-           (external_id, quelle, firma, stelle, ort, gehalt, beschreibung, quelle_url, art, status)
-         VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+           (external_id, quelle, firma, stelle, ort, gehalt, beschreibung, quelle_url, art, anzeige_datum, status)
+         VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
         [externalId, quelle, ...fields]
       );
       const row = await dbGet('SELECT * FROM jobangebote WHERE id = ?', [result.lastID]);