Notify contributors when funds are released

[kitWarse]

Summary

• Adds contributor-facing fund release notifications for milestone releases and approved withdrawals.

Issue

• Closes FEATURE: Escrow Release Notifications for Contributors #590

Changes

• Added contributor fund release notification service with in-app and email delivery.
• Added detailed fund usage email template including amount, asset, recipient, campaign link, and transaction link.
• Triggered contributor notifications after milestone releases and platform-approved withdrawals.
• Added funds_released notification preference in the settings UI.
• Added migration support for email notification preferences.

Validation

• Ran git diff --check.
• Attempted npm run lint --prefix backend; not run successfully because dependencies are not installed (eslint not found).
• Attempted npm run lint --prefix frontend; not run successfully because dependencies are not installed (eslint not found).

Notes

• Did not modify lockfiles or install dependencies.

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