Add contributor milestone voting

[kitWarse]

Summary

• Added contributor milestone approval voting so backers can approve or reject milestones while evidence is awaiting platform review.
• Added backend tally endpoints, vote persistence, and release threshold enforcement before platform release.
• Added a campaign page voting panel for pending-review milestones.

Issue

• Closes FEATURE: Contributor Voting on Milestones #586

Changes

• Added a milestone_votes table keyed by milestone and contributor.
• Added GET/POST milestone vote endpoints with contributor access checks.
• Added vote tally threshold logic to block release when contributor votes do not support approval.
• Added frontend API helpers and a contributor milestone voting panel on campaign pages.

Validation

• Ran node --check backend/src/routes/milestones.js.
• Ran git diff --check.
• Did not run full lint/test/build because dependencies are not installed in this fresh clone and the request was to PR after required changes without CI cleanup.

Notes

• Platform release remains available when no contributor votes exist.
• When votes exist, approval votes must outnumber rejection votes before release can proceed.

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