docs: update license references from MIT to AGPL-3.0

Update all marketing site references to reflect the project's AGPL-3.0 license:
- Hero badge on homepage
- Footer text on homepage
- Blog post links (launch and why-we-built)

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

blog/launch.html

@@ -123,7 +123,7 @@
     <p><strong>Links:</strong></p>
     <p>GitHub: <a href="https://github.com/groombook/groombook">https://github.com/groombook/groombook</a></p>
     <p>Infra: <a href="https://github.com/groombook/infra">https://github.com/groombook/infra</a></p>
-    <p>License: MIT</p>
+    <p>License: AGPL-3.0</p>
     <p style="margin-top: 20px; font-size: 0.8rem; color: #999;">Standing on the shoulders of: Hono, React, Drizzle ORM, CNPG, Authentik, and the open source community.</p>
   </footer>
 </body>

blog/why-we-built.html

@@ -99,7 +99,7 @@
 
   <h2>The Price</h2>
 
-  <p>Groom Book is free. MIT license. We built it because the problem is solvable and the existing solutions are exploitative.</p>
+  <p>Groom Book is free. AGPL-3.0 license. We built it because the problem is solvable and the existing solutions are exploitative.</p>
 
   <p>If you want to run it yourself, you can have it running in 5 minutes:</p>