db: add slug column to workflow table

[adityachoudhari26]

Part of #1127.

Scope

Add a slug column to the workflow table so workflows are addressable by a human-readable identifier instead of UUID. Workspace-scoped uniqueness.

Changes

• packages/db/src/schema/workflow.ts: add slug: text("slug").notNull() and a unique index on (workspace_id, slug).
• Generate Drizzle migration, then hand-edit for backwards compatibility:
  1. Add slug as nullable.
  2. Backfill from name in psql using the same slugify rules the app code will use (lowercase, spaces → -, strip URL-unsafe chars).
  3. Set notNull + add unique index.
• Pre-flight check: confirm no two existing workflow names slugify to the same value within a workspace before running the migration in prod.

Out of scope

API/UI/SDK changes — covered in sibling issues.