Phase 3 of the cross-project sqlite-migrate adoption — port nanodrop to
consume bchen-sqlite-migrate@v0.1.0. Replaces the inline db.exec(...)
block in src/db/schema.ts with applyMigrations(db, MIGRATIONS_DIR,
{ genesisProbeTable: 'users' }).
The genesis-probe (table 'users' exists) handles pre-existing prod DBs
automatically — first deploy after merge stamps 0001_init as applied
without re-executing, subsequent boots are no-ops.
Adds three npm scripts (db:migrate, db:status, db:stamp) and a
byte-stability test pinning sha256(0001_init.sql) so the migration is
treated as immutable history.
