feat(treasury): add stream preview calculator API endpoint

[Gutopro]

Closes #850

What this PR does

Adds a read-only GET /treasury/streams/preview endpoint that previews
unlocked treasury stream amounts for a beneficiary without submitting
any transaction.

Changes

apps/backend/src/treasury/dto/stream-preview.dto.ts (new)

• StreamPreviewDto — request query params (beneficiary, optional atTime)
• StreamPreviewResponseDto — response with unlocked, claimed, remaining,
  previewAt, and isActive fields

apps/backend/src/treasury/treasury.service.ts

• Added previewStream() — computes unlocked/claimed/remaining using the
  same linear-vesting formula as the on-chain contract
• Defaults atTime to current server time when omitted
• Throws TreasuryStreamNotFoundException for missing streams

apps/backend/src/treasury/treasury.controller.ts

• Added GET /treasury/streams/preview endpoint
• Full Swagger documentation with response types and error codes
• No auth guard required (read-only public endpoint)

apps/backend/src/treasury/treasury-preview.service.spec.ts (new)

• 6 tests covering:
  • Active stream returns correct unlocked/claimed/remaining
  • Fully elapsed stream returns full remaining amount
  • Zero unlocked before stream starts
  • Default atTime uses current server time
  • Not-found throws TreasuryStreamNotFoundException
  • Contract math verification test

Acceptance Criteria

• Accepts beneficiary and optional atTime inputs
• Returns unlocked, claimed, and remaining values
• Matches contract math (linear vesting formula)
• Handles not-found stream state cleanly
• 359 tests passing

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[Cedarich]

@Gutopro fix workflow