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/sqlite/002_teams.sql

@@ -23,7 +23,7 @@ CREATE TABLE IF NOT EXISTS team_members (
     id          TEXT PRIMARY KEY,
     team_id     TEXT NOT NULL REFERENCES teams(id) ON DELETE CASCADE,
     user_id     TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
-    role        TEXT NOT NULL DEFAULT 'member' CHECK (role IN ('admin', 'member')),
+    role        TEXT NOT NULL DEFAULT 'member',
     joined_at   TEXT DEFAULT (datetime('now')),
     UNIQUE(team_id, user_id)
 );