Add robust company slug for job-offer blacklist matching

Company matching relied on normText(), which kept legal-form suffixes,
spacing and umlaut spelling — so "Bosch GmbH", "Bosch AG" and "bosch gmbh"
did not match the same entry, making the blacklist unreliable.

Introduce firmaSlug(): transliterate German umlauts, strip diacritics and
trailing legal-form tokens (GmbH/AG/SE/KG/…), then kebab-join. Add a
firma_slug field to jobangebote_blacklist (schema + ALTER/backfill migration)
and match on it for typ firma/firma_stelle/auto, falling back to firma_norm
for legacy rows. POST /joboffers rejects blacklisted offers via matchBlacklist
automatically since offerSignature now carries the slug.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server.js

@@ -815,6 +815,7 @@ function initializeDatabase() {
               quelle TEXT,
               external_id TEXT,
               firma_norm TEXT,
+              firma_slug TEXT,
               stelle_norm TEXT,
               ort_norm TEXT,
               firma TEXT,
@@ -824,6 +825,25 @@ function initializeDatabase() {
               created_at DATETIME DEFAULT CURRENT_TIMESTAMP
             )
           `, () => {});
+          // Add + backfill firma_slug on pre-existing installs. The slug is a
+          // legal-form/umlaut-robust company key that the blacklist now matches
+          // on; backfill from the stored original company name so old company
+          // entries start blocking reliably too.
+          db.run('ALTER TABLE jobangebote_blacklist ADD COLUMN firma_slug TEXT', () => {
+            db.all(
+              `SELECT id, firma, firma_norm FROM jobangebote_blacklist
+               WHERE firma_slug IS NULL AND (firma IS NOT NULL OR firma_norm IS NOT NULL)`,
+              (err, rows) => {
+                if (err || !rows) return;
+                for (const r of rows) {
+                  const slug = blacklist.firmaSlug(r.firma || r.firma_norm || '');
+                  if (slug) {
+                    db.run('UPDATE jobangebote_blacklist SET firma_slug = ? WHERE id = ?', [slug, r.id], () => {});
+                  }
+                }
+              }
+            );
+          });
           // Remember the last recipient address per application (prefill).
           db.run('ALTER TABLE bewerbungen ADD COLUMN email_empfaenger TEXT', () => {});