security: medium-severity fixes (proxies, ssrf, uploads, lnurl, github_data)

- Trust the Forge reverse proxy and force https URLs in production so
  generated absolute URLs match the actual TLS termination.
- Reject Nostr profile photo URLs that aren't http(s) or that resolve to
  loopback / private (RFC1918) addresses to close an SSRF vector in
  FetchNostrProfileJob.
- Tighten image upload validation across meetup, course, and lecturer
  create/edit components: explicit mimes whitelist (jpeg, png, webp),
  max 5 MiB, and dimension cap of 4000x4000.
- Replace the silent "skip if exists" branch in LnurlAuthController with
  updateOrCreate so concurrent callers cannot race on the k1 record.
- Validate github_data on Meetup edit, decoding the JSON, and keep only
  the whitelisted keys (top, left, state) with strict type coercion to
  prevent storing arbitrary attacker-controlled JSON.

app/Http/Controllers/LnurlAuthController.php

@@ -154,14 +154,10 @@ final class LnurlAuthController extends Controller
      */
     private function ensureLoginKeyExists(string $k1, int $userId): void
     {
-        $loginKey = LoginKey::where('k1', $k1)->first();
-
-        if (! $loginKey) {
-            LoginKey::create([
-                'k1' => $k1,
-                'user_id' => $userId,
-            ]);
-        }
+        LoginKey::query()->updateOrCreate(
+            ['k1' => $k1],
+            ['user_id' => $userId],
+        );
     }
 
     /**