feat(cli): appkit db init — one-command lakebase onboarding

[ditadi]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/database/foundation [Files changed]
  • stack/database/server [Files changed]
    • stack/database/brownfield [Files changed]
      • stack/database/init [Files changed]
        • stack/database/frontend [Files changed]
          • stack/database/ui [Files changed]
            • stack/database/advanced [Files changed]
              • stack/database/release [Files changed]

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Summary

appkit db init interactively onboards an existing or new Lakebase database in one command.

• Picks/creates project, branch, endpoint
• Plans + applies env diff with .env.bak backup
• --dry-run, --from <existing>, --allow-destructive (for reset)
• Atomic .env writer
• Seed prod guard

Stack (4/8)

1. #354 foundation
2. server
3. brownfield
4. init ← this PR
5. frontend
6. ui
7. advanced
8. release

Test plan

• biome check
• typecheck
• vitest (init flow tests pass)