refactor composer to use sqlalchemy, avoid string concatenations and follow conventions

api/datasets/financial/schema_docs.yaml

@@ -9,8 +9,19 @@
 #   tables.<name>:           one-line description.
 #   columns.<table>.<col>:   one-line description. Sparse — only the ones
 #                            worth describing. Undescribed columns just have
-#                            no description in the recon.
+#                            no description in the recon. A column may also
+#                            carry a domain type, e.g.
+#                              loan.date: {desc: "...", type: date_yymmdd}
 #   relationships:           declared FKs (BIRD ships SQLite without them).
+#   encodings:               OPTIONAL. Storage-encoding overrides keyed by a
+#                            column's `type`. Each maps a role → SQL template
+#                            with a `{col}` placeholder. The composer ships
+#                            defaults (e.g. date_yymmdd); add an entry here only
+#                            to introduce a novel encoding or override a default:
+#                              encodings:
+#                                date_yymmdd:
+#                                  as_date: "TO_DATE(LPAD({col}::text, 6, '0'), 'YYMMDD')"
+#                            `financial` stores real DATE columns, so it needs none.
 
 schema: financial