feat(04-03): create JSON archival and smart retention systems

- Added ArchivalManager for JSON export/import with gzip compression
- Implemented organized directory structure by year/month
- Added batch archival operations and restore functionality
- Created RetentionPolicy with importance-based scoring
- Smart retention considers engagement, topics, user-marked importance
- MemoryManager integrates compression and archival automatically
- Added automatic compression triggering and archival scheduling
- Comprehensive archival statistics and retention recommendations
- Support for backup integration and restore verification

src/memory/backup/__init__.py

@@ -0,0 +1,11 @@
+"""
+Memory backup and archival subsystem.
+
+This package provides conversation archival, retention policies,
+and long-term storage management for the memory system.
+"""
+
+from .archival import ArchivalManager
+from .retention import RetentionPolicy
+
+__all__ = ["ArchivalManager", "RetentionPolicy"]