feat(sprint-5.5): store block metadata (version, changelog) in PostgreSQL

- Prisma schema: added `changelog Json @default("[]")` to Block model
- Migration: 20260324141120_add_changelog_field
- Seed: 8 blocks with actual versions (v1.0–v1.2) and changelog entries
- API: PATCH /blocks/by-path accepts changelog field
- CORS: accept any localhost port (regex pattern)
- BlockChangelog component: renders version history from API or fallback
- BlockMetaBar: loads changelog from API, passes to BlockChangelog
  - Removed "API офлайн" text, replaced with subtle gray dot
  - Added defaultChangelog prop for offline fallback
- Block pages: removed hardcoded changelog JSX, use defaultChangelog prop
- Updated SPRINTS.md with completed tasks

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/api/prisma/schema.prisma

@@ -46,6 +46,7 @@ model Block {
   name        String
   version     String
   isInPreview Boolean  @default(false)
+  changelog   Json     @default("[]")
   updatedAt   DateTime @updatedAt
   createdAt   DateTime @default(now())
 }