Implemented comprehensive session lifecycle management:
- SessionManager class with LRU eviction policy
- Automatic cleanup of expired sessions
- Connection pooling and reuse
- Memory leak prevention

Features:
- LRU cache with configurable max_sessions limit
- Automatic eviction of least recently used sessions
- TTL-based session expiration (default: 1 hour)
- Background cleanup task (default: 5 minutes interval)
- Thread-safe session access with asyncio.Lock
- Statistics tracking (cache hits, misses, evictions)

Session Lifecycle:
1. get_session() - Get or create session (lazy initialization)
2. Session pooling - Reuse existing sessions
3. LRU eviction - Remove oldest when at capacity
4. TTL expiration - Cleanup inactive sessions
5. Graceful shutdown - Close all sessions on shutdown

Memory Leak Prevention:
- Proper connection cleanup via clear_session()
- Automatic eviction prevents unbounded growth
- Background task removes expired sessions
- Shutdown closes all active sessions

Integration:
- Added to DIContainer with lazy initialization
- Integrated into initialize() and shutdown() lifecycle
- Configurable via ErniConfig.data_dir

Configuration:
- max_sessions: 100 (prevents memory exhaustion)
- session_ttl: 3600 seconds (1 hour)
- cleanup_interval: 300 seconds (5 minutes)
- db_path: data/sessions.db (persistent storage)

Benefits:
- Prevents memory leaks from unclosed connections
- Reduces database connection overhead
- Improves performance with session reuse
- Automatic cleanup of stale sessions
- Thread-safe concurrent access

Tests:
- 13 comprehensive unit tests
- Test LRU eviction, TTL expiration, cleanup
- Test concurrent access, statistics
- All tests passing (13/13)

Before:
- Sessions created but never cleaned up
- Database connections left open
- Memory usage grows unbounded
- No session reuse

After:
- Automatic session lifecycle management
- Proper connection cleanup
- Bounded memory usage (max 100 sessions)
- Session pooling and reuse
- Background cleanup task

Resolves: openai#11 (HIGH priority)
Impact: Eliminates memory leaks and improves stability