Migrate all apps to defpage declarative page routes

Replace Python GET page handlers with declarative defpage definitions in .sx
files across all 8 apps (sx docs, orders, account, market, cart, federation,
events, blog). Each app now has sxc/pages/ with setup functions, layout
registrations, page helpers, and .sx defpage declarations.

Core infrastructure: add g I/O primitive, PageDef support for auth/layout/
data/content/filter/aside/menu slots, post_author auth level, and custom
layout registration. Remove ~1400 lines of render_*_page/render_*_oob
boilerplate. Update all endpoint references in routes, sx_components, and
templates to defpage_* naming.

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

federation/app.py

@@ -84,11 +84,20 @@ def create_app() -> "Quart":
         app.jinja_loader,
     ])
 
+    # --- defpage setup ---
+    from sxc.pages import setup_federation_pages
+    setup_federation_pages()
+
     # --- blueprints ---
     # Well-known + actors (webfinger, inbox, outbox, etc.) are now handled
     # by the shared AP blueprint registered in create_base_app().
     app.register_blueprint(register_identity_bp())
-    app.register_blueprint(register_social_bp())
+
+    social_bp = register_social_bp()
+    from shared.sx.pages import mount_pages
+    mount_pages(social_bp, "federation")
+    app.register_blueprint(social_bp)
+
     app.register_blueprint(register_fragments())
 
     # --- home page ---