svcforge: reference implementation

Complete working build of the system learn-python/ teaches.
164 tests, mypy --strict clean, domain coverage 99%.

migrations/003_day2.sql

@@ -0,0 +1,16 @@
+-- 003_day2.sql — the whole of day 2: two columns and one query.
+--
+-- Forward-only, expand/contract. Both additions are nullable / defaulted, so the old
+-- code keeps running against the new schema between the migrate and the deploy. That
+-- ordering is not optional: migrate first, deploy second, and a column the running
+-- code has never heard of must not break it.
+
+alter table instances add column maintenance_window text;   -- '0 3 * * 0|Asia/Ho_Chi_Minh', null = any time
+
+-- One row per service type, one column that matters. `halted` is what stops a bad
+-- chart after the first tenant instead of after all of them, and it is cleared by
+-- hand with SQL — an automatic un-halt would just resume breaking things.
+create table catalog_versions (
+  service_type  text primary key,
+  rollout_state text not null default 'ok'    -- 'ok' | 'halted'
+);