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/sqlite/016_team_user_roles.sql

@@ -0,0 +1,17 @@
+-- ==========================================
+-- Armature — 016 Team User Roles (many-to-many, SQLite)
+-- ==========================================
+
+CREATE TABLE IF NOT EXISTS team_user_roles (
+    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,
+    assigned_by TEXT REFERENCES users(id) ON DELETE SET NULL,
+    assigned_at TEXT DEFAULT (datetime('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);