## Summary
Mirror the real-estate historical-balance pattern for loan accounts.
When a loan is created with a current balance, linearly interpolate
monthly `account_balances` rows between the loan start date
(`original_amount`) and today (current balance).
## Approach
Since we don't know how the user actually paid down the loan, use simple
linear regression between two known points:
- `(start_date, original_amount)` from `loan_details`
- `(today, current_balance)` from the latest `account_balances` row
Rows are placed on the start date, the 1st of each intermediate month,
and today.
## Changes
- **`LoanBalanceGeneratorService`** — linear interpolation + upsert on
`(account_id, balance_date)` so existing rows aren't duplicated.
- **`GenerateHistoricalLoanBalancesJob`** — `ShouldQueue`, 3 tries, 10s
backoff. Takes account, original amount, start date, current balance,
from, to.
- **`Settings/AccountController::store`** (loan branch) — after creating
`loanDetail`, if a starting balance was provided: generate the last 12
months synchronously, dispatch older window to the queue when
`start_date` predates it.
- **`LoanDetailController::update`** — when the detail is first created
for an existing account, use the most recent `AccountBalance` as the
current value and apply the same sync/async split.
## Tests
- Service: interpolation, single-balance edge, future start_date, upsert
dedupe, monthly 1st placement, from/to range.
- Job: implements `ShouldQueue`, respects from/to range.
```
Tests: 8 passed (62 assertions)
```
No dependency or directory-structure changes.
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