REST API: implement documented-but-missing endpoints

[fuseraft]

docs/reference/api.md documents several endpoints that don't exist yet.

Near-term (explicitly called out in the "Additional Endpoints Planned" note):

• DELETE /api/concepts/{id} — delete a concept
• GET /api/types — list concept types
• GET /api/tags — list all tags
• GET /api/health — health check endpoint

Longer-term ("Future Enhancements" section):

• Batch operations — create/update multiple concepts in one request
• Relationships endpoint — query concept relationships
• History endpoint — view concept change history (via git)
• Export endpoint — export filtered concepts as JSON/CSV
• Webhook support — notify on concept changes
• GraphQL API — alternative to REST for complex queries

Suggest tackling the near-term group first (small, self-contained); the longer-term group can be split into follow-up issues as they're picked up.