Add blog feature and fix broken test/build infra

BlogController was an empty stub with no view, so /blog 500'd on every
visit. Adds a real Post model, public listing/detail pages, and an admin
CRUD screen mirroring the existing Exercise admin pattern (image upload
or URL, auto-slug, publish toggle). Seeds 5 real posts so it's not empty
on launch.

Also fixes two things that were silently broken: phpunit.xml pointed
DB_DATABASE at a non-existent "testing" file with no DB_CONNECTION set,
and the stock ExampleTest had RefreshDatabase commented out - both meant
the test suite never actually ran successfully.

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

phpunit.xml

@@ -22,7 +22,8 @@
         <env name="APP_MAINTENANCE_DRIVER" value="file"/>
         <env name="BCRYPT_ROUNDS" value="4"/>
         <env name="CACHE_STORE" value="array"/>
-        <env name="DB_DATABASE" value="testing"/>
+        <env name="DB_CONNECTION" value="sqlite"/>
+        <env name="DB_DATABASE" value=":memory:"/>
         <env name="MAIL_MAILER" value="array"/>
         <env name="QUEUE_CONNECTION" value="sync"/>
         <env name="SESSION_DRIVER" value="array"/>