feat: add Patentprozesskostenrechner fee calculation engine + API

Pure Go implementation of patent litigation cost calculator with:
- Step-based GKG/RVG fee accumulator across 4 historical schedules (2005/2013/2021/2025 + Aktuell alias)
- Instance multiplier tables for 8 court types (LG, OLG, BGH NZB/Rev, BPatG, BGH Null, DPMA, BPatG Canc)
- Full attorney fee calculation (VG, TG, Erhöhungsgebühr Nr. 1008 VV RVG, Auslagenpauschale)
- Prozesskostensicherheit computation
- UPC fee data (pre-2026 and 2026 schedules with value-based brackets, recoverable costs ceilings)
- Public API: POST /api/fees/calculate, GET /api/fees/schedules (no auth required)
- 22 unit tests covering all calculation paths

Fixes 3 Excel bugs:
- Bug 1: Prozesskostensicherheit VAT formula (subtract → add)
- Bug 2: Security for costs uses GKG base for court fee, not RVG
- Bug 3: Expert fees included in BPatG instance total

backend/internal/router/router.go

@@ -82,6 +82,12 @@ func New(db *sqlx.DB, authMW *auth.Middleware, cfg *config.Config, calDAVSvc *se
 	billingH := handlers.NewBillingRateHandler(billingRateSvc)
 	templateH := handlers.NewTemplateHandler(templateSvc, caseSvc, partySvc, deadlineSvc, tenantSvc)
 
+	// Fee calculator (public — no auth required, pure computation)
+	feeCalc := services.NewFeeCalculator()
+	feeCalcH := handlers.NewFeeCalculatorHandler(feeCalc)
+	mux.HandleFunc("POST /api/fees/calculate", feeCalcH.Calculate)
+	mux.HandleFunc("GET /api/fees/schedules", feeCalcH.Schedules)
+
 	// Public routes
 	mux.HandleFunc("GET /health", handleHealth(db))