feat(ledger): serialize api money amounts as decimal strings

[tiana-code]

What

Money amounts were serialized over the API as JSON numbers. JSON has one numeric type and JS clients parse it into float64, truncating large or high-scale NUMERIC(38,18) values before any client code runs - a ledger correctness hazard.

This serializes money amounts as JSON decimal strings via BigDecimal.toPlainString() (exact digits, preserved scale, never scientific notation):

before  {"amount": 100.00}
after   {"amount": "100.00"}

Scope

• MoneyAmountSerializer (JsonSerializer<BigDecimal>)
• applied to BalanceResponse.amount (the only money output field today) + @Schema(type=string, format=decimal) so generated OpenAPI is accurate
• per-field annotation, not a global Jackson module: outbox/event/idempotency serialization stays byte-stable
• input unchanged: default BigDecimal deserialization already accepts string and number

Tests

• MoneyAmountSerializerTest: trailing zeros preserved (100.00 -> "100.00"); high-scale value 12345678901234567890.123456789012345678 round-trips losslessly with no exponent
• AccountControllerTest balance assertion updated to the string form (validates the real Spring ObjectMapper path)

Unblocks Sandbox UI Screen 2 (Account Detail + Balance) on a lossless string amount. ADR: Money over JSON (prepared for Wiki).

Closes #113