Sync W3DS identities and automate platform slugs

[coodos]

Summary

• enrich W3DS logins from AaaS with the newest person display name and avatar while keeping authentication available when AaaS fails
• render account identifiers as cosmetic @enames across the UI without changing routes, Git remotes, mentions, forms, or API values
• remove editable repository/platform slug fields and derive collision-safe slugs from the display name for both new-platform and existing-application flows
• document production AaaS configuration and the automatic slug behavior

Safety

• AaaS access stays server-side and uses a bounded timeout
• avatar downloads validate HTTP(S), size, and an external-host allowlist to prevent private-network SSRF
• profile enrichment is best-effort and preserves the OIDC profile on lookup failure

Verification

• full Go package compilation with SQLite tags
• focused model, settings, W3DS, auth, repository, template, and issue tests
• onboarding integration tests for platform creation and existing-application porting
• frontend lint and frontend test suite
• Go vet and branch-scoped GolangCI lint (0 issues)
• changed documentation markdown lint and git diff validation