Add additional capital deposit flow (3d-4)

Deposits credit a fund's available cash directly: Domain gains
CapitalPolicy (positive, two-decimal, overflow-checked amounts plus a
settle function that never touches units, NAV or realized P&L).
Persistence records deposits in fund_capital_deposits with a
fund_capital_idempotencies table, crediting cash and the deposit row
atomically under the same replay/conflict semantics as orders. The
API exposes POST /funds/{id}/capital/deposit and GET
/funds/{id}/capital/deposits behind the bearer token and idempotency
keys. The frontend adds an '追加资金' entry on the fund cash card with
in-place balance refresh, and repeats of the same amount reuse the
idempotency key so double submits credit cash exactly once. Browser
QA extends the J-series (J12-J18): invalid-amount rejection without
POST, single credited deposit with a 32-hex key, instant balance
refresh, unchanged units/cost proving no realized-income impact, and
same-key replay crediting only once.

1 files changed, 27 insertions, 0 deletions

diff --git a/src/FundLab.Domain/Capital.fs b/src/FundLab.Domain/Capital.fs
new file mode 100644
index 0000000..b2ae697
--- /dev/null
+++ b/src/FundLab.Domain/Capital.fs
@@ -0,0 +1,27 @@
+namespace FundLab.Domain
+
+open System
+
+module CapitalPolicy =
+    let cashMaximum = 999999999999999999.99m
+
+    let validateDeposit (amount: decimal) : Result<unit, string> =
+        if amount <= 0m then
+            Error "deposit amount must be positive"
+        elif Decimal.Round(amount, 2) <> amount then
+            Error "deposit amount exceeds cash precision"
+        elif amount > cashMaximum then
+            Error "deposit amount exceeds database precision"
+        else
+            Ok ()
+
+    let settle (availableCash: decimal) (amount: decimal) : Result<decimal, string> =
+        match validateDeposit amount with
+        | Error message -> Error message
+        | Ok() ->
+            let updated = availableCash + amount
+
+            if updated > cashMaximum then
+                Error "available cash exceeds database precision"
+            else
+                Ok updated