feat: add phase 1 mem db (#10634)

- 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.

codex-rs/state/src/model/mod.rs

@@ -1,9 +1,11 @@
 mod log;
+mod thread_memory;
 mod thread_metadata;
 
 pub use log::LogEntry;
 pub use log::LogQuery;
 pub use log::LogRow;
+pub use thread_memory::ThreadMemory;
 pub use thread_metadata::Anchor;
 pub use thread_metadata::BackfillStats;
 pub use thread_metadata::ExtractionOutcome;
@@ -12,6 +14,7 @@ pub use thread_metadata::ThreadMetadata;
 pub use thread_metadata::ThreadMetadataBuilder;
 pub use thread_metadata::ThreadsPage;
 
+pub(crate) use thread_memory::ThreadMemoryRow;
 pub(crate) use thread_metadata::ThreadRow;
 pub(crate) use thread_metadata::anchor_from_item;
 pub(crate) use thread_metadata::datetime_to_epoch_seconds;