fix(settlement): per-currency netting, balance guards, Decimal rates

Settlement correctness cluster from CODE-REVIEW-2026-06 (#2, #8, #13)
plus libra-#38:

- format_net_settlement_entry now enforces the same inline balance
  constraint as the fiat formatter (payment = receivable - payable
  + credit) and grows an optional credit leg. An unbalanced
  settlement raises instead of reaching the ledger.
- on_invoice_paid settles only what the payment covers: a partial
  payment clears that much receivable; excess (or a payment with
  nothing owed) becomes user credit. Previously the full prior
  balance was cleared against a smaller payment, shipping unbalanced
  postings. Settlement links are attached only when the payment
  clears the full open balance, and only for same-currency entries.
- get_unsettled_entries_bql returns each entry's real posting
  currency (was hardcoded "EUR") and exact Decimal amount strings
  (was float). /receivables/settle nets only entries denominated in
  the settlement currency.
- fiat_rate/btc_rate metadata computed via Decimal (new
  fiat_rate_metadata helper) instead of float division — cost-basis
  records no longer carry float drift.
- format_posting_at_average_cost omits the cost braces when
  cost_currency is unset ("SATS {}" is invalid Beancount).
- Underpay error payload serializes amounts as exact Decimal strings.
- validate_metadata catches decimal.InvalidOperation so bad
  fiat_amount input becomes ValidationError (libra-#38); flipped the
  tracking xfail.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

fava_client.py

@@ -1817,7 +1817,7 @@ class FavaClient:
             # Query 1: Get all original expense/receivable entries for this user
             # These are entries with the expense-entry or receivable-entry tag
             original_query = f"""
-                SELECT date, narration, account, number, weight, links,
+                SELECT date, narration, account, number, currency, weight, links,
                        any_meta('entry-id') as entry_id
                 WHERE account ~ '{account_pattern}'
                   AND '{entry_tag}' IN tags
@@ -1851,7 +1851,10 @@ class FavaClient:
             entries_by_link: Dict[str, Dict[str, Any]] = {}
 
             for row in original_result["rows"]:
-                date_val, narration, account, number, weight, links, entry_id = row
+                (
+                    date_val, narration, account, number, currency,
+                    weight, links, entry_id,
+                ) = row
 
                 # Skip if no links
                 if not links or not isinstance(links, list):
@@ -1875,9 +1878,11 @@ class FavaClient:
                 if entry_link in entries_by_link:
                     continue
 
-                # Parse amounts
-                fiat_amount = abs(float(number)) if number else 0.0
-                fiat_currency = "EUR"  # Default, could be extracted from posting
+                # Parse amounts. The posting's real currency matters: callers
+                # net these totals per currency, and the old hardcoded "EUR"
+                # let USD (or SATS-only) entries be summed as if they were EUR.
+                fiat_amount = str(abs(Decimal(str(number)))) if number else "0"
+                fiat_currency = currency or "EUR"
 
                 # Parse SATS from weight column
                 sats_amount = 0