bug: affiliate conversions accept fractional sat amounts

[absalonCRC]

Bug

Manual affiliate conversions can be created or edited with fractional sale_amount_sats values, for example 100.5. Satoshi amounts are indivisible, so accepting decimals can produce inconsistent sale and commission records.

Impact

Sellers can store impossible affiliate conversion amounts through the manual conversion APIs. A later commission recalculation may also run on a fractional satoshi value.

Expected

POST /api/affiliates/offers/[id]/conversions and PUT /api/affiliates/offers/[id]/conversions should accept only positive integer satoshi amounts.

Fix

Validate sale_amount_sats with integer checks before recording or recalculating conversions, and add regression coverage for fractional create/update requests.

Payment for the active uGig affiliate testing bounty can go to SOL: 27sdMYXofqoM9qR13bZhccRNYeEgYn5EoHXTSJn4QWKP.

[nguyenlnp]

/attempt #139

Submitted PR #165: #165

Scope:

• POST and PUT handlers now reject non-integer sale_amount_sats
• calculateCommission() throws on non-integer input (defense-in-depth)
• 7 new regression tests covering fractional, negative, zero, and edge cases

All 18 tests pass. ⭐ Starred.