chore: migrate repository URLs from AnsoCode/Sencho to studio-saelix/sencho

Updates all hardcoded GitHub repository references across 21 files:
- package.json: repository URL, bugs URL, homepage, description, author
- CONTRIBUTING.md: bug report template URL
- SECURITY.md: advisory URL, cosign cert-identity regexp
- .github/CODEOWNERS: @AnsoCode -> @studio-saelix/maintainers
- .github/workflows/ci.yml: repositories scope (Sencho -> sencho), docs-sync git URL
- .github/workflows/cla.yml: path-to-document URL
- .github/workflows/docker-publish.yml: cosign verify comment
- frontend/**/*.tsx: issues and changelog links (3 components)
- frontend/public/.well-known/security.txt: Contact and Policy URLs
- security/vex/sencho.openvex.json: @id field
- docs/openapi.yaml: license URL
- docs/docs.json: navbar and footer GitHub links (5 instances)
- docs/security.mdx: advisory and SECURITY.md links
- docs/reference/verifying-images.mdx: repo link + cosign regexp + legacy identity note
- docs/reference/contact.mdx: issues, LICENSE, advisory, policy, CoC links
- docs/reference/security-advisories.mdx: releases link
- docs/operations/verifying-images.mdx: cosign regexps and VEX download URL (6 instances)
- docs/operations/upgrade.mdx: releases links (2 instances)
- backend/src/utils/version-check.ts: GitHub Releases API endpoint

CHANGELOG.md intentionally excluded (release-please managed).
Legacy cosign identity note added for pre-migration image verification.

backend/src/utils/version-check.ts

@@ -8,7 +8,7 @@ import { CacheService } from '../services/CacheService';
  */
 
 async function fetchFromGitHub(): Promise<string | null> {
-  const res = await fetch('https://api.github.com/repos/AnsoCode/Sencho/releases/latest', {
+  const res = await fetch('https://api.github.com/repos/studio-saelix/sencho/releases/latest', {
     headers: { Accept: 'application/vnd.github+json', 'User-Agent': 'Sencho' },
     signal: AbortSignal.timeout(10000),
   });