migrate(): rewrite the two historical event records

[0xdevcollins]

The record layout change breaks decoding of existing rows. Mainnet holds exactly two events, both Completed with zero remaining escrow, so a one-pass rewrite is feasible now and never will be again.

Done when

• migrate() rewrites both records from percentages to equivalent prize_floors, using each event's own total_budget
• Both remain readable via get_event after migrate
• Per-(from, to) dispatch follows the existing pattern in admin.rs

Events: 271539957145796609, 271539957145796610. See plan 2.1 and 3.4.

---

Plan: boundless-bounty-programs-implementation-plan.md

[coderabbitai]

🔗 Related PRs

#81 - chore: strip explanatory comments from contract source [merged]
#83 - feat(events): replace push-based select_winners with pull-model claim_prize for unlimited winners [open]
#90 - feat(events): pull-model prize claims for Single-release events [merged]
#96 - fix(events): namespace OpSeen by authorizing caller (follow-up to #68/#95) [merged]
#99 - feat(events): stop enforcing the submission deadline on-chain [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!