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.

docs/reference/contact.mdx

@@ -11,7 +11,7 @@ Sencho provides dedicated email channels for different types of inquiries. Use t
   Technical support, how-to questions, and troubleshooting help. Available to Skipper and Admiral license holders via **Settings > Help & Support** in the app.
 </Card>
 
-Community users can get help through the [Documentation](https://docs.sencho.io) and [GitHub Issues](https://github.com/AnsoCode/Sencho/issues).
+Community users can get help through the [Documentation](https://docs.sencho.io) and [GitHub Issues](https://github.com/studio-saelix/sencho/issues).
 
 ## General inquiries
 
@@ -26,7 +26,7 @@ Community users can get help through the [Documentation](https://docs.sencho.io)
 </Card>
 
 This address appears in:
-- The [LICENSE](https://github.com/AnsoCode/Sencho/blob/main/LICENSE) file for alternative licensing inquiries
+- The [LICENSE](https://github.com/studio-saelix/sencho/blob/main/LICENSE) file for alternative licensing inquiries
 - Upgrade prompts inside the app (paywall screens)
 - The [Refund Policy](https://sencho.io/refund) and pricing section on the website
 
@@ -36,9 +36,9 @@ This address appears in:
   Vulnerability reports, security incidents, and bug bounty submissions. **Do not open a public issue for security vulnerabilities.**
 </Card>
 
-You can also use [GitHub private vulnerability reporting](https://github.com/AnsoCode/Sencho/security/advisories/new). Both the app and the website publish a `/.well-known/security.txt` file that points to this address.
+You can also use [GitHub private vulnerability reporting](https://github.com/studio-saelix/sencho/security/advisories/new). Both the app and the website publish a `/.well-known/security.txt` file that points to this address.
 
-See the full [Security Policy](https://github.com/AnsoCode/Sencho/blob/main/SECURITY.md) for response times and disclosure guidelines.
+See the full [Security Policy](https://github.com/studio-saelix/sencho/blob/main/SECURITY.md) for response times and disclosure guidelines.
 
 ## Privacy
 
@@ -54,7 +54,7 @@ This address is listed in the [Privacy Policy](https://sencho.io/privacy) and [T
   Reports of Code of Conduct violations in any Sencho community space.
 </Card>
 
-All reports are reviewed promptly and handled with confidentiality. See the [Code of Conduct](https://github.com/AnsoCode/Sencho/blob/main/CODE_OF_CONDUCT.md) for details.
+All reports are reviewed promptly and handled with confidentiality. See the [Code of Conduct](https://github.com/studio-saelix/sencho/blob/main/CODE_OF_CONDUCT.md) for details.
 
 ## Summary