fix(db): add missing 'supervisor' value to the user_role enum at startup

CREATE TYPE ... EXCEPTION WHEN duplicate_object no-ops on existing types, so
volumes created before 'supervisor' existed kept an enum without it and
assigning the role 500'd: invalid input value for enum user_role: "supervisor".
ALTER TYPE ... ADD VALUE IF NOT EXISTS is idempotent and safe on any volume.

backend/app/db/init_db.py

@@ -420,6 +420,9 @@ async def ensure_rbac_schema(engine: AsyncEngine) -> None:
     volume — the RBAC model expects these and login 500s without them."""
     stmts = (
         "ALTER TABLE users ADD COLUMN IF NOT EXISTS username TEXT",
+        # The live user_role enum predates 'supervisor' — CREATE TYPE no-ops on
+        # an existing type, so without this ALTER, assigning the role 500s.
+        "ALTER TYPE user_role ADD VALUE IF NOT EXISTS 'supervisor'",
         "ALTER TABLE projects ADD COLUMN IF NOT EXISTS location TEXT DEFAULT ''",
         "ALTER TABLE projects ADD COLUMN IF NOT EXISTS project_manager TEXT DEFAULT ''",
         "ALTER TABLE projects ADD COLUMN IF NOT EXISTS planned_start DATE",