Epic: materialised P&L and Balance Sheet report shapes

[avandenberghe]

Context

The Trial Balance report already groups accounts into Balance Sheet (Asset + Liability + Equity) and Profit & Loss (Revenue + Expense) blocks with subtotals — so the data is there. What's missing is a "give me the report in the shape an accountant expects to read it" view: a dedicated Balance Sheet and Profit & Loss sub-mode under Reports.

These aren't new queries — they're reshapings of get_trial_balance() output:

• Balance Sheet = Assets (DR-natural) + Liabilities (CR-natural) + Equity (CR-natural). Two-column form: Assets on the left, Liabilities + Equity on the right, each side summing to the same total. As-of date filter (already in trial balance).
• Profit & Loss = Revenue (CR-natural) − Expense (DR-natural) = Net Income. Top section revenue, bottom section expenses, then net at the bottom. Date range filter (from / to), unlike trial balance which is point-in-time.

Why it earns its keep

Visual / cognitive — a forum admin sees "Balance Sheet" and "P&L" and recognises them; "Trial Balance" they have to reverse-engineer. Same data, more legible framing.

Implementation shape

• Two new Reports sub-modes: balance_sheet, profit_loss.
• Two new controller methods that call get_trial_balance() (existing) + reshape.
• Two new template files (or branches in acp_bbaccounts_reports.html).
• Both ACP and FE Reports get them (FE = main_controller, mirror).
• For the FE, gated by the same u_accounts_view permission that already gates the rest of the FE Reports — no new perm.

Out of scope (separate epic)

• Cash flow statement — needs additional metadata bbAccounts doesn't currently track (operating vs investing vs financing classification per entry/account). Skip for now.

Depends on

Nothing. Could ship before #7 (period locking) and avatharbe/bbAccounts-dev#88 (close) since it only consumes existing service surface.

Acceptance

• Two new Reports sub-modes: balance_sheet, profit_loss.
• Balance Sheet shows Assets vs (Liabilities + Equity) with point-in-time date.
• P&L shows Revenue − Expenses → Net Income with date-range filter.
• Both surfaces in ACP and FE.
• Multi-pool aware: one BS / P&L per currency pool, like trial balance does today.
• No new ledger service methods.