feat: P2 backend completion — 19 endpoints across 5 modules

[ULookup]

Summary

P2 backend completion: 19 REST endpoints for Inspector panel. All synchronous CRUD, directly calling existing Store methods.

Module	Endpoints	Key Changes
B. Pipeline	2	PipelineManager::retreat_to_phase with allowed_retreat validation
C. Diary/Memory/Relations/Voice	8	diary get/patch/search, memory summary list/get, relation upsert, voice
D. World Export/Import	2	full JSON snapshot (share-snapshot-v1), import with ID remapping
E. Knowledge Graph	4	entity list, entity relations, search, scene extraction result
F. Timeline	3	timeline list, event get, time advance

Changes

• libs/http/ — WorldbuildingHttpHandler: 19 new handlers
• libs/worldbuilding/ — PipelineManager, WorldbuildingService, NarrativeStore: 5 new methods
• docs/api-reference.md — 5 new API sections, endpoint count 104 → 123

Test plan

• Build: cmake --build build
• World export/import ID remapping
• Timeline forward-only validation
• KG entity filtering by type