Feat v0.3.4 team roles + multi-party validation (#18)

Custom team roles: removed CHECK constraint on team_members.role,
roles stored in teams.settings, roles API, stage_config.required_role
enforced on claim. Multi-party signoff: workflow_signoffs table,
validation gate in advanceInternal, SubmitSignoff engine method,
signoff HTTP API. Frontend: dynamic role management, stage config
validation UI, signoff panel. Design docs for extension lifecycle
and trigger composition. 20 store tests passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/database/migrations/postgres/002_teams.sql

@@ -22,8 +22,7 @@ CREATE TABLE IF NOT EXISTS team_members (
     id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
     team_id     UUID NOT NULL REFERENCES teams(id) ON DELETE CASCADE,
     user_id     UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
-    role        VARCHAR(20) NOT NULL DEFAULT 'member'
-                CHECK (role IN ('admin', 'member')),
+    role        VARCHAR(50) NOT NULL DEFAULT 'member',
     joined_at   TIMESTAMPTZ DEFAULT NOW(),
     UNIQUE(team_id, user_id)
 );