Drop the retired release tables and discord_delivery (after the rebuild ships)

Rían approved dropping the release tables (2026-10-18). This must ship after the rebuild → main cutover (#37), as its own release, so a rollback to the pre-rebuild images still works during the cutover.

Remove:

• Tables: release and release_issue (V15), plus discord_delivery (V34). The rebuild no longer reads or writes discord_delivery; see PAR-420 and PAR-424.
• Backend:
  • the read-only releases domain: ReleasesFacade, the two deprecated GETs, the 410 write endpoints, and ReleaseMcpTools (tasklie_list_releases, tasklie_get_release, tasklie_move_releases, plus the retired write tools);
  • the DORA fallback that counts frozen releases;
  • the DeleteProject releases guard;
  • the /releases → Deployments redirects in the web app, which can stay or go (keep them if they're cheap).
• Migration: one idempotent Flyway migration (drop table if exists release_issue; drop table if exists release; drop table if exists discord_delivery;) using the next free number.
• Spec: regenerate the OpenAPI spec and the web schema.

Before merging: confirm production has run the rebuild for a while and that nothing still references these tables (git grep).

State reconciliation — 2026-10-19

PR #81 is still open and draft, not merged. Implementation is underway, so status is In progress; table retirement is not complete on main.