feat: add point-in-time stream snapshots and claim endpoint

[InfamousCanary]

Summary

This PR completes the existing stream snapshot and claim API endpoints.

GET /api/streams/:id/snapshot?at=

Adds point-in-time stream state calculation for an arbitrary Unix timestamp.

The endpoint now:

• validates the at query parameter as a Unix timestamp
• computes stream status at the requested point in time
• returns vested, claimable, and remaining amounts
• accounts for claim events that occurred on or before the requested timestamp
• excludes later stream state changes from historical snapshots
• freezes historical calculation appropriately for canceled streams
• returns 400 for invalid, missing, or negative timestamps
• returns 404 for unknown streams

This covers the required snapshot behavior, including:

• before stream start → scheduled, vested 0
• at stream end → completed, vested equals the total stream amount

POST /api/streams/:id/claim

Completes the existing claim endpoint by replacing the placeholder local transaction result with an actual Soroban claim transaction flow.

The endpoint now:

• restricts claiming to the authenticated stream recipient
• rejects completed or canceled streams
• obtains the current on-chain claimable amount
• builds the contract claim(stream_id, recipient, amount) invocation
• prepares the Soroban transaction through RPC
• signs it using the configured backend signing key
• submits the transaction and waits for confirmation
• returns the real transaction hash and amount claimed
• records the successful claim in event history with the transaction hash and ledger sequence

No new dependencies or database migrations were introduced.

Validation

Targeted validation completed successfully:

• ESLint passed for all changed/new implementation and test files
• isolated TypeScript check passed for the new Soroban claim service
• GET /snapshot tests: 7/7 passed
• POST /claim tests: 7/7 passed
• targeted tests total: 14/14 passed
• scoped git diff --check passed

The repository-wide TypeScript check is currently blocked by pre-existing syntax errors in backend/src/services/streamStore.ts on the upstream baseline; those unrelated errors were intentionally left unchanged.

Closes #705
Closes #710

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