feat(fusion_accounting_reports): currency conversion service

Pure-Python helper for FX conversion at report end-date. Handles direct
rates, inverse rates, and fallback to most-recent-rate-on-or-before.
fetch_rates() pulls from res.currency.rate using the same
1/rate inversion convention Odoo uses internally.

Made-with: Cursor

fusion_accounting_reports/services/__init__.py

@@ -1,3 +1,4 @@
 from . import date_periods
 from . import account_hierarchy
 from . import totaling
+from . import currency_conversion

fusion_accounting_reports/services/currency_conversion.py

@@ -0,0 +1,66 @@
+"""Multi-currency conversion for financial reports.
+
+Converts move-line amounts to the report's display currency at the
+report end-date. Pure-Python - caller provides exchange rates as a
+dict {(source_code, target_code, date): rate}."""
+
+from dataclasses import dataclass
+from datetime import date
+
+
+@dataclass
+class ConversionRate:
+    source: str
+    target: str
+    rate: float
+    rate_date: date
+
+
+def convert_amount(amount: float, *, source_currency: str, target_currency: str,
+                   rate_date: date, rates: dict) -> float:
+    """Convert `amount` from source to target at the given date.
+
+    `rates` is a dict keyed by (source, target, date) -> rate.
+    If source == target, returns amount unchanged."""
+    if source_currency == target_currency:
+        return amount
+    key = (source_currency, target_currency, rate_date)
+    if key in rates:
+        return amount * rates[key]
+    inv_key = (target_currency, source_currency, rate_date)
+    if inv_key in rates:
+        inv = rates[inv_key]
+        if inv != 0:
+            return amount / inv
+    candidates = [
+        (d, r) for (s, t, d), r in rates.items()
+        if s == source_currency and t == target_currency and d <= rate_date
+    ]
+    if candidates:
+        candidates.sort(key=lambda x: x[0], reverse=True)
+        return amount * candidates[0][1]
+    raise ValueError(
+        f"No exchange rate available for {source_currency}->{target_currency} on or before {rate_date}"
+    )
+
+
+def fetch_rates(env, *, target_currency_id: int, as_of: date,
+                source_currency_ids: list[int] | None = None) -> dict:
+    """Fetch all relevant rates from res.currency.rate as of a given date.
+
+    Returns the dict-of-rates structure consumed by convert_amount.
+    Pulls only rates where source != target and date <= as_of."""
+    Rate = env['res.currency.rate'].sudo()
+    target = env['res.currency'].browse(target_currency_id)
+    domain = [
+        ('name', '<=', as_of),
+        ('currency_id', '!=', target.id),
+    ]
+    if source_currency_ids:
+        domain.append(('currency_id', 'in', source_currency_ids))
+    rates_recs = Rate.search(domain)
+
+    out = {}
+    for r in rates_recs:
+        out[(r.currency_id.name, target.name, r.name)] = (1.0 / r.rate) if r.rate else 0.0
+    return out