feat: implement batch_contribute for TreasuryContract

[JHON12091986]

Closes #2
/claim #2

Changes

• Added batch_contribute function to TreasuryContract
• Allows admin to process multiple member contributions in one transaction
• Skips non-members with skipped_non_member event
• Emits batch_contribution summary event
• Includes member authorization for each transfer
• Added comprehensive tests

Testing

• ✅ Full batch: all members processed
• ✅ Partial skip: non-members skipped
• ✅ Empty batch: no changes
• ✅ Unauthorized: non-admin rejected

All 29 tests passed.

Payment wallet (USDC on Ethereum): 0x46243c949f30280771c0675Bc8A16155A9B96e51