chore: improve build deadline code (#19203)

- Adds/improves a lot of comments to make the autostop calculation code
clearer
- Changes the behavior of the enterprise template schedule store to
match the behavior of the workspace TTL endpoint when the new TTL is
zero
- Fixes a bug in the workspace TTL endpoint where it could unset the
build deadline, even though a max_deadline was specified
- Adds a new constraint to the workspace_builds table that enforces the
deadline is non-zero and below the max_deadline if it is set
- Adds CHECK constraint enum generation to scripts/dbgen, used for
testing the above constraint
- Adds Dean and Danielle as CODEOWNERS for the autostop calculation code

coderd/database/migrations/000356_enforce_deadline_below_max_deadline.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE workspace_builds
+    DROP CONSTRAINT workspace_builds_deadline_below_max_deadline;

coderd/database/migrations/000356_enforce_deadline_below_max_deadline.up.sql

@@ -0,0 +1,20 @@
+-- New constraint: (deadline IS NOT zero AND deadline <= max_deadline) UNLESS max_deadline is zero.
+-- Unfortunately, "zero" here means `time.Time{}`...
+
+-- Update previous builds that would fail this new constraint. This matches the
+-- intended behaviour of the autostop algorithm.
+UPDATE
+    workspace_builds
+SET
+    deadline = max_deadline
+WHERE
+    deadline > max_deadline
+    AND max_deadline != '0001-01-01 00:00:00+00';
+
+-- Add the new constraint.
+ALTER TABLE workspace_builds
+    ADD CONSTRAINT workspace_builds_deadline_below_max_deadline
+        CHECK (
+            (deadline != '0001-01-01 00:00:00+00'::timestamptz AND deadline <= max_deadline)
+            OR max_deadline = '0001-01-01 00:00:00+00'::timestamptz
+        );