docs(m3): place create decisions in the contracts

- DATA-MODEL.md: storage engine (stdlib sqlite3 + schema.sql, no ORM/migrations),
  pragmas, per-test isolation.
- API.md: revision coupling, async job model, PUT check order, server-derived ready.
- PROMPTS.md: no prompt files ship; the DB is the only prompt source.

docs/contracts/DATA-MODEL.md

@@ -35,6 +35,11 @@ Status: SPEC · 2026-09-18. Store: SQLite.
 - Photo bytes live in `slab_photos` BLOBs; the `/images` volume is not a second store. It holds a
   health-probe file and short-lived staging for 1024 px Call-1 JPEGs and Woo media uploads, purged
   after use.
+- The store is SQLite through the stdlib `sqlite3` module and a checked-in `schema.sql`
+  (`CREATE TABLE IF NOT EXISTS`); there is no ORM and no migration framework. Pragmas are
+  `journal_mode=WAL`, `foreign_keys=ON`, `busy_timeout`, `synchronous=NORMAL`, with one
+  process-wide connection behind a lock. The database path and image directory are explicit so a
+  test runs against isolated storage.
 
 ## 2. `slabs`