feat: add DBML schema files and relspecgo migration generation

- Add schema/*.dbml covering all existing tables (001-019)
- Wire relspecgo via make generate-migrations target
- Add make check-schema-drift for CI drift detection
- Add schema/README.md documenting the DBML-first workflow

Closes #19

schema/README.md

@@ -0,0 +1,35 @@
+# Schema workflow
+
+The `schema/*.dbml` files are the database schema source of truth.
+
+## Generate SQL migrations
+
+Run:
+
+```bash
+make generate-migrations
+```
+
+This uses `relspec` to convert the DBML files into PostgreSQL SQL and writes the generated schema migration to:
+
+- `migrations/020_generated_schema.sql`
+
+## Check schema drift
+
+Run:
+
+```bash
+make check-schema-drift
+```
+
+This regenerates the SQL from `schema/*.dbml` and compares it with `migrations/020_generated_schema.sql`.
+If the generated output differs, the command fails so CI can catch schema drift.
+
+## Workflow
+
+1. Update the DBML files in `schema/`
+2. Run `make generate-migrations`
+3. Review the generated SQL
+4. Commit both the DBML changes and the generated migration
+
+Existing handwritten migrations stay in place. Going forward, update the DBML first and regenerate the SQL from there.