feat: add detailed logging for module loading and context providers

Add INFO-level logging to help diagnose module loading issues:

- discovery.py: Log summary of discovered modules by tier (core/optional/internal)
- service.py: Log which modules are enabled for each platform (DEBUG level)
- page_context.py: Log context building with platform info and which
  modules contributed context with key counts

Example log output:
  [MODULES] Auto-discovered 18 modules: 5 core, 11 optional, 2 internal
  [CONTEXT] Building PLATFORM context for platform 'main' with 5 enabled modules
  [CONTEXT] Context providers called: cms(3 keys), billing(3 keys)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/service.py

@@ -182,6 +182,11 @@ class ModuleService:
         # Resolve dependencies - add required modules
         enabled_set = self._resolve_dependencies(enabled_set)
 
+        logger.debug(
+            f"[MODULES] Platform '{platform.code}' (id={platform_id}) has "
+            f"{len(enabled_set)} modules enabled: {sorted(enabled_set)}"
+        )
+
         return enabled_set
 
     def _migrate_json_to_junction_table(