feat: initialize blog app with blueprints and templates

Extract blog-specific code from the coop monolith into a standalone
repository. Includes auth, blog, post, admin, menu_items, snippets
blueprints, associated templates, Dockerfile (APP_MODULE=app:app),
entrypoint, and Gitea CI workflow.

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

bp/auth/services/__init__.py

@@ -0,0 +1,24 @@
+from .login_redirect import pop_login_redirect_target, store_login_redirect_target
+from .auth_operations import (
+    get_app_host,
+    get_app_root,
+    send_magic_email,
+    load_user_by_id,
+    find_or_create_user,
+    create_magic_link,
+    validate_magic_link,
+    validate_email,
+)
+
+__all__ = [
+    "pop_login_redirect_target",
+    "store_login_redirect_target",
+    "get_app_host",
+    "get_app_root",
+    "send_magic_email",
+    "load_user_by_id",
+    "find_or_create_user",
+    "create_magic_link",
+    "validate_magic_link",
+    "validate_email",
+]