Add OpenCorporates collector for org targets, gated on an API token

Closes the last coverage gap for org (person remains intentionally
unaddressed — no good free/non-scraping option exists). Unlike the
other 8 sources, OpenCorporates now requires an API token even on its
free tier (confirmed: keyless request gets 401). RunnerRegistry::has()
checks services.opencorporates.token is configured before exposing the
source as runnable, so it degrades to "not yet integrated" in the UI
rather than a broken Run button, until OPENCORPORATES_API_TOKEN is set.

Also fixes a real bug hit while building this: a RunnerRegistry::REQUIRES
class constant mapping source key to a closure crashed on file load
(PHP class constants can't hold closures) — inlined the one check
instead of building an abstraction for a single case.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

database/seeders/SourceSeeder.php

@@ -70,6 +70,13 @@ class SourceSeeder extends Seeder
                 'target_types' => ['domain'],
                 'runner' => 'dns:records',
             ],
+            [
+                'key' => 'opencorporates',
+                'name' => 'OpenCorporates',
+                'description' => 'Company registration search by name. Requires a free OpenCorporates API token (OPENCORPORATES_API_TOKEN) — hidden from "runnable" sources until one is configured.',
+                'target_types' => ['org'],
+                'runner' => 'http:opencorporates',
+            ],
         ];
 
         foreach ($sources as $source) {