[codex] Grant signup credits

[brandonkachen]

Summary

• Grant new Codebuff accounts a one-time 500-credit signup bonus during the NextAuth create-user flow.
• Preserve monthly free credits for grandfathered users by requiring a prior monthly free grant history.
• Update pricing, usage, and out-of-credits copy so new users see signup-credit messaging instead of monthly free-credit messaging.

Validation

• bun --cwd packages/billing test src/__tests__/grant-credits.test.ts with required test env vars
• bun --cwd packages/billing typecheck
• bun --cwd web typecheck
• bun --cwd cli typecheck
• git diff --check