fix(bug 12): P&L report honours from_date/to_date across multiple periods

[roger296]

Summary

getProfitAndLoss() in `src/engine/reports.ts` was applying a hard single-period filter:

```typescript
.where('transaction_lines.period_id', opts.period_id)
```

which silently neutralised the `from_date`/`to_date` filters further down. Any caller passing a multi-period date range got back only the single period named in `period_id` — the date parameters were accepted but had no effect.

Fix

Make the `period_id` filter conditional — apply it only when no date range is supplied. When `from_date` or `to_date` is present, the existing date filters select across periods as the caller expects. This mirrors the pattern already used by `getBalanceSheet()` in the same file.

Tests

Added 4 new integration tests covering:

• default single-period behaviour still works (regression guard)
• date range spanning two periods aggregates revenue from both
• date range confined to a different period than `period_id` returns that period's data (confirms `period_id` is ignored once dates are set)
• aggregate across range equals sum of individual monthly reports

Test plan

• `npx tsc --noEmit` clean
• Unit tests: 205/205 passing
• Integration tests: 363/363 passing (4 new)
• Post-deploy: call `gl_get_profit_and_loss` with `from_date`/`to_date` spanning multiple periods (e.g. Feb 2025 → Nov 2025) and confirm aggregated totals

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