Add LinkedIn OAuth flow

- linkedin_auth API: generates auth URL with CSRF state stored in session
- linkedin_callback: exchanges code for tokens, writes to project's config.json
- index.php: route /linkedin/callback before auth check
- Connect button wired up in JS; reads LINKEDIN_CLIENT_ID/SECRET from server env

Server setup required:
  SetEnv LINKEDIN_CLIENT_ID ...
  SetEnv LINKEDIN_CLIENT_SECRET ...
Redirect URI to register in LinkedIn app: https://<host>/linkedin/callback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

views/project/_tab_botconfig.php

@@ -40,10 +40,9 @@
         <div class="small fw-semibold" id="liConnectionLabel">Checking…</div>
         <div class="text-muted small" id="liTokenExpiry"></div>
       </div>
-      <a href="#" class="btn btn-sm btn-outline-primary ms-auto disabled" id="liConnectBtn">
+      <button class="btn btn-sm btn-outline-primary ms-auto" id="liConnectBtn">
         <i class="bi bi-box-arrow-in-right me-1"></i>Connect LinkedIn
-        <span class="badge bg-secondary ms-1">coming soon</span>
-      </a>
+      </button>
     </div>
   </div>