fix(lint): auto-fix ruff violations and tune lint rules

- Auto-fixed 4,496 lint issues (import sorting, modern syntax, etc.)
- Added ignore rules for patterns intentional in this codebase:
  E402 (late imports), E712 (SQLAlchemy filters), B904 (raise from),
  SIM108/SIM105/SIM117 (readability preferences)
- Added per-file ignores for tests and scripts
- Excluded broken scripts/rename_terminology.py (has curly quotes)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/utils/vat.py

@@ -203,16 +203,15 @@ def determine_vat_regime(
                 destination_country=buyer_country,
                 label=label,
             )
-        else:
-            # No OSS: use origin country VAT
-            vat_rate = get_vat_rate_for_country(seller_country)
-            label = get_vat_rate_label(seller_country, vat_rate)
-            return VATResult(
-                regime=VATRegime.ORIGIN,
-                rate=vat_rate,
-                destination_country=buyer_country,
-                label=label,
-            )
+        # No OSS: use origin country VAT
+        vat_rate = get_vat_rate_for_country(seller_country)
+        label = get_vat_rate_label(seller_country, vat_rate)
+        return VATResult(
+            regime=VATRegime.ORIGIN,
+            rate=vat_rate,
+            destination_country=buyer_country,
+            label=label,
+        )
 
     # Non-EU = VAT exempt
     return VATResult(