Add Prune API skeleton

[zipdoki]

Summary

This implements API skeleton for pruning the edges.

One endpoint covers every prunable structure (CACHE now, GROUP later):
POST /graph/v3/databases/{database}/tables/{table}/edges/prune

$ curl -X POST "https://.../graph/v3/databases/mydb/tables/mytable/edges/prune" \
  -H "Content-Type: application/json" \
  -d '{
    "type": "cache",
    "targets": [
      { "start": "user:1001", "direction": "OUT" }
    ]
  }'

{
  "results": [
    {
      "start": "user:1001",
      "direction": "OUT",
      "cache": "permission_created_at_desc",
      "status": "PRUNED"
    }
  ]
}

Part of #322.

Changes

• Add prune payloads to core.edge.payload
• Add POST .../edges/prune to EdgeMutationController
• Add EdgeCachePruneSpec (engine) and EdgeCachePruneE2ETest (server); behavior cases disabled until the data plane lands

How to Test

./gradlew :engine:test --tests "*.EdgeCachePruneSpec" :server:test --tests "*.EdgeCachePruneE2ETest"

AI Assistance

• This PR was written largely with AI assistance.
  • Tool / model: Claude Code (Opus 4.8)