test: add form config validator and HTTP integration test

test-config: static YAML validator for post-form.md — checks that the
add-page-by-form action name, pageconfig/pagefrontmatter blocks, and all
required fields are correctly wired. Fast, no server needed. Catches the
class of bug that caused silent post failures.

test-post: end-to-end HTTP test — logs in, submits the form, verifies an
entry.md was created on disk, then cleans up. Requires GRAV_TEST_USER and
GRAV_TEST_PASS in .env (see .env.example).

  make test-config   # fast, no credentials needed
  make test-post     # full e2e, needs .env credentials
  make test          # both

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

.env.example

@@ -19,3 +19,8 @@ MAIN_REPO=https://gitea.example.com/org/travel-blog-intotheeast.git
 GITEA_HOST=gitea.example.com
 GITEA_USER=deploy-user
 GITEA_TOKEN=your-gitea-personal-access-token
+
+# Test credentials — used by 'make test-post' (must be a valid Grav site login user)
+GRAV_TEST_USER=mischa
+GRAV_TEST_PASS=your-grav-password
+GRAV_BASE_URL=http://localhost:8081