feat: add phase 1 mem db

[jif-oai]

• Schema: thread_id (PK, FK to threads.id with cascade delete), trace_summary, memory_summary, updated_at.
• Migration: creates the table and an index on (updated_at DESC, thread_id DESC) for efficient recent-first reads.
• Runtime API (DB-only):
  • get_thread_memory(thread_id): fetch one memory row.
  • upsert_thread_memory(thread_id, trace_summary, memory_summary): insert/update by thread id and always advance updated_at.
  • get_last_n_thread_memories_for_cwd(cwd, n): join thread_memory with threads and return newest n rows for an exact cwd match.
• Model layer: introduced ThreadMemory and row conversion types to keep query decoding typed and consistent with existing state models.