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/handlers/fee_calculator.go

@@ -0,0 +1,53 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/models"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+// FeeCalculatorHandler handles fee calculation API endpoints.
+type FeeCalculatorHandler struct {
+	calc *services.FeeCalculator
+}
+
+func NewFeeCalculatorHandler(calc *services.FeeCalculator) *FeeCalculatorHandler {
+	return &FeeCalculatorHandler{calc: calc}
+}
+
+// Calculate handles POST /api/fees/calculate.
+func (h *FeeCalculatorHandler) Calculate(w http.ResponseWriter, r *http.Request) {
+	var req models.FeeCalculateRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if req.Streitwert <= 0 {
+		writeError(w, http.StatusBadRequest, "streitwert must be positive")
+		return
+	}
+	if req.VATRate < 0 || req.VATRate > 1 {
+		writeError(w, http.StatusBadRequest, "vat_rate must be between 0 and 1")
+		return
+	}
+	if len(req.Instances) == 0 {
+		writeError(w, http.StatusBadRequest, "at least one instance is required")
+		return
+	}
+
+	resp, err := h.calc.CalculateFullLitigation(req)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	writeJSON(w, http.StatusOK, resp)
+}
+
+// Schedules handles GET /api/fees/schedules.
+func (h *FeeCalculatorHandler) Schedules(w http.ResponseWriter, r *http.Request) {
+	writeJSON(w, http.StatusOK, h.calc.GetSchedules())
+}