feat: Implement real Ed25519 sweep authorization signing

[khaadish]

Closes #141

Summary

Replaces the placeholder sweep authorization signature implementation with real Ed25519 signing backed by an authorized signer private key loaded from environment configuration.

Note: This PR must remain synchronized with bridgelet-core Issue #69, which defines the canonical signing payload format consumed by verify_sweep_authorization().

Also: update

SWEEP_SIGNING_KEY_SEED=f76f684a3a8b64f32a7dc7eba0b0a5040ba66b5ea67dad348c3b69b79db3339c

in production env to avoid code breakage

Changes

• Replaced stubbed generateAuthSignature() implementation
• Added dedicated signing utility under src/common/crypto
• Added environment configuration and startup validation for the authorized signer private key
• Removed development/test-only environment guard
• Updated sweep authorization flow to generate real signatures
• Documented required environment variables in .env.example

Security

• Signing key is sourced exclusively from environment configuration
• No hardcoded signing keys or fallback values
• Private key material is never logged or exposed
• Signing logic is isolated for auditing and future replacement

Files

Modified

• src/modules/sweeps/providers/contract.provider.ts
• src/config/*
• .env.example

Added

• src/common/crypto/*

Acceptance Criteria

• generateAuthSignature() produces a valid Ed25519 signature
• Signature is verifiable by the on-chain contract
• Environment guard has been removed
• Authorized signer private key is validated at startup
• No hardcoded signing key exists in the codebase
• Sweeps initiated through the SDK are accepted by the contract and update state successfully

Dependencies

• Requires the canonical payload format defined in bridgelet-core Issue Test: Replace commented tests with proper integration tests (#43) #69
• Must be deployed alongside the corresponding on-chain verification implementation

[khaadish]

The contract.spec.ts is stale now that the authorisation has been upgraded. Should I go ahead and update the test too?

[phertyameen]

Yes please. Go ahead and edit it appropraitely. Carefull with the test intentst though.