feat: federate FableLoom stories

[atomantic]

Summary

• Add opt-in per-record FableLoom federation with full-mirror backfill and card-level peer controls.
• Replicate story graphs through schema-gated LWW merges with conflict recovery, tombstone propagation, and scene image/video assets.
• Surface the FableLoom category in Instances and document the federated storage contract.

Test plan

• cd server && npm test -- services/fableLoom/records.test.js routes/fableLoom.test.js routes/peerSync.test.js routes/instances.test.js services/instances.test.js services/sharing/peerSync.test.js services/sharing/peerSyncAssets.test.js services/sharing/integrity.test.js services/sharing/tombstoneGc.test.js services/sharing/privacyNeverFederates.test.js services/conflictJournalResolver.test.js services/dataSync.pipelineUniverse.test.js lib/schemaVersions.test.js lib/syncWire.test.js lib/db/schema/schema.test.js lib/apiRouteCatalog.test.js lib/socketEventCatalog.test.js ../scripts/migrations/_shippedHashDrift.test.js
• cd client && npm test -- src/services/apiPeerSync.test.js src/components/sharing/SyncToPeerButton.test.jsx src/pages/FableLoom.test.jsx
• cd client && npm run lint
• cd client && npm run build
• npm run generate:api-docs