feat(llm): refresh current model registry

[k002bill2]

Summary

• Add the verified current provider models claude-opus-5, claude-fable-5-1, gemini-3.8-flash, and gpt-6-astra to the central registry.
• Update Claude Sonnet 5 pricing and synchronize proxy/session/external usage pricing mirrors.
• Synchronize dashboard offline fallbacks and regression coverage.
• Keep new models non-default until provider smoke/canary validation; preserve existing DB/admin defaults.

System model

• AOS remains LLM_PROVIDER=codex_cli.
• The current Codex system configuration already uses gpt-6-astra.
• No runtime restart, database mutation, deployment, or provider default promotion was performed by this PR.

Validation

• Backend full suite: 1985 passed, 32 skipped (5 pre-existing warnings).
• Backend registry/pricing focus: 60 passed.
• Frontend full suite: 4495 passed.
• Frontend settings focus: 47 passed.
• Frontend type-check, lint, production build: passed.
• Ruff: passed.
• git diff --check: passed.

Notes

• Merge and deployment are intentionally not performed in this task.