REL-04 supervised server acceptance

Repository h8v6/heyaira; branch codex/server-supervised-execution. Base d3dd8fc5fa501d06c6c6295c9a08dbc80a98b201.

Implemented additive migration 018 and an explicit HTTP execution contract: operator-authored pinned plans; project/node/account fences; current task version check; atomic single claim; 90-second renewable lease with 15-minute hard deadline; cancellation; consecutive idempotent events; terminal work receipts that do not mark a task done or claim independent verification.

The whole suite ran against fresh disposable PostgreSQL 16 databases: 248 passed, zero skipped. New real HTTP tests exercise authorization, cross-project denial, task changes, concurrent claim, renewal, expiry, cancellation, revoked binding, event replay/conflicts, rejected raw-output fields, receipt uniqueness, and unchanged task completion status.

Generated MCP contract remains current (18 tools); documentation contract and strict MkDocs passed. Existing clients require no migration of their payloads. Migration is additive and old application code ignores the new tables.

The opt-in Bridge process acceptance is tracked separately in REL-05/06. Production rollout remains gated on exact-commit CI and REL-07 acceptance.