Feat v0.9.3 team user roles (#77)

Co-authored-by: Jeffrey Smith <jasafpro@gmail.com>
Co-committed-by: Jeffrey Smith <jasafpro@gmail.com>

server/database/migrations/postgres/016_team_user_roles.sql

@@ -0,0 +1,17 @@
+-- ==========================================
+-- Armature — 016 Team User Roles (many-to-many)
+-- ==========================================
+
+CREATE TABLE IF NOT EXISTS team_user_roles (
+    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(50) NOT NULL,
+    assigned_by UUID REFERENCES users(id) ON DELETE SET NULL,
+    assigned_at TIMESTAMPTZ DEFAULT NOW(),
+    UNIQUE(team_id, user_id, role)
+);
+
+CREATE INDEX IF NOT EXISTS idx_team_user_roles_team_user ON team_user_roles(team_id, user_id);
+CREATE INDEX IF NOT EXISTS idx_team_user_roles_team ON team_user_roles(team_id);
+CREATE INDEX IF NOT EXISTS idx_team_user_roles_user ON team_user_roles(user_id);