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/docs.json

@@ -29,7 +29,7 @@
     "links": [
       {
         "type": "github",
-        "href": "https://github.com/AnsoCode/Sencho"
+        "href": "https://github.com/studio-saelix/sencho"
       }
     ],
     "primary": {
@@ -40,7 +40,7 @@
   },
   "footer": {
     "socials": {
-      "github": "https://github.com/AnsoCode/Sencho"
+      "github": "https://github.com/studio-saelix/sencho"
     },
     "links": [
       {
@@ -48,14 +48,14 @@
         "items": [
           { "label": "Website", "href": "https://sencho.io" },
           { "label": "Pricing", "href": "https://sencho.io/#pricing" },
-          { "label": "Changelog", "href": "https://github.com/AnsoCode/Sencho/releases" }
+          { "label": "Changelog", "href": "https://github.com/studio-saelix/sencho/releases" }
         ]
       },
       {
         "header": "Community",
         "items": [
-          { "label": "GitHub", "href": "https://github.com/AnsoCode/Sencho" },
-          { "label": "Contributing", "href": "https://github.com/AnsoCode/Sencho/blob/main/CONTRIBUTING.md" }
+          { "label": "GitHub", "href": "https://github.com/studio-saelix/sencho" },
+          { "label": "Contributing", "href": "https://github.com/studio-saelix/sencho/blob/main/CONTRIBUTING.md" }
         ]
       },
       {