feat: event emitted on supplier tier change

[shakurJJ]

Summary

Supplier tier (Bronze/Silver/Gold) changes silently — there's no event for off-chain systems to react to a supplier's tier changing.

Proposed Behaviour

• Emit a supplier_tier_changed event whenever a supplier's computed tier changes (old tier, new tier, supplier address).
• Evaluated at the same points the tier is already computed today (e.g. on shipment creation / reputation update), no new computation paths added.

Acceptance Criteria

• Event is emitted only when the tier actually changes, not on every evaluation
• Event payload includes old tier, new tier, and supplier address
• Tests cover an upgrade, a downgrade, and a no-change evaluation (no event)

[Udokwuebube]

@Udokwuebube has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hi, i would like to work on this.

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[MJ-RWA]

@MJ-RWA has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello shakurJJ, Can l work on this to implement this fix?
ETA: 4 Hours

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[oscarj007]

@oscarj007 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer
I would love to take up this issue it is something I have prior experience on, I will deliver.

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[Victorjonah-prog]

@Victorjonah-prog has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hi maintainer, id love to work on this issue

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