feat: add deadline CRUD, calculator, and holiday services (Phase 1C)

- Holiday service with German federal holidays, Easter calculation, DB loading
- Deadline calculator adapted from youpc.org (duration calc + non-working day adjustment)
- Deadline CRUD service (tenant-scoped: list, create, update, complete, delete)
- Deadline rule service (list, filter by proceeding type, hierarchical rule trees)
- HTTP handlers for all endpoints with tenant resolution via X-Tenant-ID header
- Router wired with all new endpoints under /api/
- Tests for holiday and calculator services (8 passing)

backend/internal/handlers/calculate.go

@@ -0,0 +1,89 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+	"time"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/models"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+// CalculateHandlers holds handlers for deadline calculation endpoints
+type CalculateHandlers struct {
+	calculator *services.DeadlineCalculator
+	rules      *services.DeadlineRuleService
+}
+
+// NewCalculateHandlers creates calculate handlers
+func NewCalculateHandlers(calc *services.DeadlineCalculator, rules *services.DeadlineRuleService) *CalculateHandlers {
+	return &CalculateHandlers{calculator: calc, rules: rules}
+}
+
+// CalculateRequest is the input for POST /api/deadlines/calculate
+type CalculateRequest struct {
+	ProceedingType   string   `json:"proceeding_type"`
+	TriggerEventDate string   `json:"trigger_event_date"`
+	SelectedRuleIDs  []string `json:"selected_rule_ids,omitempty"`
+}
+
+// Calculate handles POST /api/deadlines/calculate
+func (h *CalculateHandlers) Calculate(w http.ResponseWriter, r *http.Request) {
+	var req CalculateRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if req.ProceedingType == "" || req.TriggerEventDate == "" {
+		writeError(w, http.StatusBadRequest, "proceeding_type and trigger_event_date are required")
+		return
+	}
+
+	eventDate, err := time.Parse("2006-01-02", req.TriggerEventDate)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid trigger_event_date format, expected YYYY-MM-DD")
+		return
+	}
+
+	var results []services.CalculatedDeadline
+
+	if len(req.SelectedRuleIDs) > 0 {
+		ruleModels, err := h.rules.GetByIDs(req.SelectedRuleIDs)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to fetch selected rules")
+			return
+		}
+		results = h.calculator.CalculateFromRules(eventDate, ruleModels)
+	} else {
+		tree, err := h.rules.GetRuleTree(req.ProceedingType)
+		if err != nil {
+			writeError(w, http.StatusBadRequest, "unknown proceeding type")
+			return
+		}
+		// Flatten tree to get all rule models
+		var flatNodes []services.RuleTreeNode
+		flattenTree(tree, &flatNodes)
+
+		ruleModels := make([]models.DeadlineRule, 0, len(flatNodes))
+		for _, node := range flatNodes {
+			ruleModels = append(ruleModels, node.DeadlineRule)
+		}
+		results = h.calculator.CalculateFromRules(eventDate, ruleModels)
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"proceeding_type":    req.ProceedingType,
+		"trigger_event_date": req.TriggerEventDate,
+		"deadlines":          results,
+	})
+}
+
+func flattenTree(nodes []services.RuleTreeNode, result *[]services.RuleTreeNode) {
+	for _, n := range nodes {
+		*result = append(*result, n)
+		if len(n.Children) > 0 {
+			flattenTree(n.Children, result)
+		}
+	}
+}