api: workflow slug support (create, lookup, run, patch)

[adityachoudhari26]

Part of #1127. Depends on #1130.

Scope

Add slug as a first-class workflow identifier across the API.

Changes

• POST workflow: accept optional slug field. If absent, derive from name (lowercase, spaces → -, strip URL-unsafe chars). On collision within a workspace, return an error that includes both the conflicting slug and the existing workflow's UUID so CLI scripts can recover.
• GET by slug: GET /v1/workspaces/:workspaceId/workflows/slug/:slug (matches existing by-name path-param convention).
• Run by slug: trigger endpoint accepts the slug in the path in addition to UUID.
• PATCH workflow: allow changing slug; enforce the same workspace-scoped uniqueness check.
• Shared slugify helper used by both the migration backfill and the API.
• Validation on user-supplied slugs (allowed characters, length). Reserved-words handling deferred.

Out of scope

UI, CLI, SDK, Terraform — covered in sibling issues.