Add a job-offer blacklist with URL-based de-duplication

Offers are now de-duplicated by normalized URL (tracking params stripped)
in addition to (quelle, external_id), so the same posting never lands
twice — even re-scraped under a new id. Deleting an offer (web or API)
auto-blacklists it, so it can never reappear.

lib/blacklist.js provides shared normalization + matching. Manual entries
can block a URL, a whole domain, a company, or a company+title posting
(gender-marker tolerant). New /blacklist page lists and manages entries.

REST API: GET/POST /joboffers/blacklist, DELETE /joboffers/blacklist/{id};
POST /joboffers returns 409 when blacklisted; DELETE /joboffers/{id}
auto-blacklists (opt out with ?blacklist=false). Swagger updated with the
new paths and schemas.

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

views/partials/header.ejs

@@ -32,6 +32,16 @@
                     <span id="jobangeboteBadge" class="hidden absolute -top-1 -right-1 min-w-[18px] h-[18px] px-1 flex items-center justify-center rounded-full bg-green-500 text-white text-[10px] font-bold ring-2 ring-blue-800 dark:ring-gray-900">0</span>
                 </a>
 
+                <!-- Blacklist (blocked job offers) link -->
+                <a href="/blacklist"
+                   class="flex items-center gap-1.5 px-3 py-2 rounded-md bg-white/20 hover:bg-white/30 transition-colors text-white text-sm font-medium"
+                   title="Geblockte Jobangebote (Blacklist) verwalten">
+                    <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M18.364 18.364A9 9 0 005.636 5.636m12.728 12.728A9 9 0 015.636 5.636m12.728 12.728L5.636 5.636"></path>
+                    </svg>
+                    <span class="hidden sm:inline">Blacklist</span>
+                </a>
+
                 <!-- Vorlagen (base documents) link -->
                 <a href="/vorlagen"
                    class="flex items-center gap-1.5 px-3 py-2 rounded-md bg-white/20 hover:bg-white/30 transition-colors text-white text-sm font-medium"