0010 - add session persistence models and storage coverage

[ethanbailie]

This PR introduces durable session persistence primitives before implementing session orchestration and API lifecycle behaviour.

What Changed

• Added session domain models in tako_vm/models.py
  • SessionRecord
  • SessionEvent
  • SessionStatus and direction literals
• Added migration 0002_sessions in tako_vm/storage.py
  • sessions table
  • session_events table with dedupe index (session_id, file_name)
• Added storage operations in tako_vm/storage.py
  • save/get/list/touch session
  • save/list session events
  • cursor pagination support
• Added row mappers for new session entities

Tests Added/Updated

• tests/test_models.py
  • model defaults and literal validation
• tests/test_storage.py
  • CRUD/list filters
  • touch timestamp updates
  • event dedupe behavior
  • cursor-based event listing
• tests/test_postgres_integration.py
  • migration includes 0002_sessions

How To Review

1. Review new models in tako_vm/models.py.
2. Review migration DDL and indexes in tako_vm/storage.py.
3. Review storage methods for idempotence and cursor semantics.
4. Validate durability via tests/test_storage.py (behavior-focused, low mocking).

Suggested Verification

• ruff check tako_vm tests
• pytest tests/test_models.py tests/test_storage.py tests/test_postgres_integration.py -v

Out of Scope

• No session container lifecycle yet.
• No session HTTP endpoints yet.