fix: OpenRouter API key save fails with network error

The 0004_add_openrouter_provider.sql migration existed but was never
registered in _journal.json, so the 'openrouter' value was missing from
the api_key_provider PostgreSQL enum. Inserting an OpenRouter key threw
a DB error that was unhandled, causing Next.js to return an HTML 500;
the frontend's res.json() then threw, showing "Netzwerkfehler".

Fixes:
- Add 0004_add_openrouter_provider to _journal.json (idx 7) so the
  migration runs on next deploy and registers 'openrouter' in the enum
- Fix null-label duplicate check: use isNull() instead of passing
  undefined to and(), which incorrectly matched all provider keys
- Wrap DB insert in try/catch to return a proper JSON error instead of
  crashing with an unhandled exception

Co-Authored-By: Paperclip <noreply@paperclip.ing>

drizzle/meta/_journal.json

@@ -50,6 +50,13 @@
       "when": 1776451200000,
       "tag": "0006_seed_system_skills_fix",
       "breakpoints": true
+    },
+    {
+      "idx": 7,
+      "version": "7",
+      "when": 1776537600000,
+      "tag": "0004_add_openrouter_provider",
+      "breakpoints": true
     }
   ]
 }