Implement production-ready TrustScore backend

[debarghya131]

Objective

Implement a reliable backend TrustScore system for SkillBridge.

Requirements

• Calculate scores from verified server-side events.
• Support credits from Skill Hub, completed GIGs, approved practice, and quality reviews.
• Support penalties with 90-day expiry.
• Prevent duplicate, invalid, future, or manually forged events.
• Apply tier ceilings from 0 to 1000.
• Reconcile cached scores from the event ledger.
• Protect concurrent updates.
• Expose score breakdown, activity, opportunities, penalties, and evidence requirements.
• Add automated tests for all scoring rules.

Acceptance Criteria

• Students cannot award TrustScore points themselves.
• Duplicate evidence cannot increase the score.
• Invalid or future evidence is ignored.
• Public profiles show the correct current score.
• All backend tests pass.