[I-016] [Tech Debt]: Standardize API route versioning #372

Priority: Medium
Phase: Phase 4 (API Polish)
Dependencies: #371

📝 User Story

As an API consumer,
I want to have consistent API versioning,
So that I know which endpoints are stable and can plan for version migration.

🎯 Scope & Requirements

• Currently, some routes are /api/collection, /api/entities, /api/workflows (unversioned) while others are /api/v1/entities, /api/v1/admin/me (versioned)
• Decide on a versioning strategy: either all routes under /api/v1/ or keep internal admin routes unversioned
• Document which routes are "internal admin" vs. "public API"
• Add deprecation headers for any routes that will be moved
• Ensure the API key-authenticated routes (/api/v1/*) are clearly separated from JWT-authenticated routes

✅ Acceptance Criteria

• Given the API route table, When reviewed, Then every route has a clear versioning rationale documented.
• Given the public API, When an external consumer reads the docs, Then all public routes are under /api/v1/.

📋 Task Checklist

• Audit all routes and categorize (admin vs. public)
• Migrate routes to consistent versioning
• Add deprecation headers for old routes
• Update API documentation