feat(m27): certificate export (PEM/PKCS#12) and S/MIME EKU support

Add certificate export in PEM (JSON or file download) and PKCS#12 formats.
Private keys are never included — they stay on agents. Add EKU-aware
issuance threading profile EKUs (serverAuth, clientAuth, codeSigning,
emailProtection, timeStamping) through the full issuance pipeline. Fix
agent CSR SAN splitting for email addresses, adaptive KeyUsage flags for
S/MIME vs TLS, and a pre-existing generateID collision bug in deployment
job creation.

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

internal/api/handler/export.go

@@ -0,0 +1,132 @@
+package handler
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"strings"
+
+	"github.com/shankar0123/certctl/internal/api/middleware"
+	"github.com/shankar0123/certctl/internal/service"
+)
+
+// ExportService defines the service interface for certificate export operations.
+type ExportService interface {
+	ExportPEM(ctx context.Context, certID string) (*service.ExportPEMResult, error)
+	ExportPKCS12(ctx context.Context, certID string, password string) ([]byte, error)
+}
+
+// ExportHandler handles HTTP requests for certificate export operations.
+type ExportHandler struct {
+	svc ExportService
+}
+
+// NewExportHandler creates a new ExportHandler with a service dependency.
+func NewExportHandler(svc ExportService) ExportHandler {
+	return ExportHandler{svc: svc}
+}
+
+// ExportPEM exports a certificate and its chain in PEM format.
+// GET /api/v1/certificates/{id}/export/pem
+func (h ExportHandler) ExportPEM(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		Error(w, http.StatusMethodNotAllowed, "Method not allowed")
+		return
+	}
+
+	requestID := middleware.GetRequestID(r.Context())
+
+	// Extract certificate ID from path: /api/v1/certificates/{id}/export/pem
+	id := extractCertIDFromExportPath(r.URL.Path)
+	if id == "" {
+		ErrorWithRequestID(w, http.StatusBadRequest, "Certificate ID is required", requestID)
+		return
+	}
+
+	result, err := h.svc.ExportPEM(r.Context(), id)
+	if err != nil {
+		if strings.Contains(err.Error(), "not found") {
+			ErrorWithRequestID(w, http.StatusNotFound, "Certificate not found", requestID)
+			return
+		}
+		ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to export certificate", requestID)
+		return
+	}
+
+	// Check if client wants file download via Accept header or ?download=true query param
+	if r.URL.Query().Get("download") == "true" {
+		w.Header().Set("Content-Type", "application/x-pem-file")
+		w.Header().Set("Content-Disposition", "attachment; filename=\"certificate.pem\"")
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(result.FullPEM))
+		return
+	}
+
+	JSON(w, http.StatusOK, result)
+}
+
+// ExportPKCS12 exports a certificate and chain in PKCS#12 format.
+// POST /api/v1/certificates/{id}/export/pkcs12
+// Body: { "password": "optional-password" }
+func (h ExportHandler) ExportPKCS12(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		Error(w, http.StatusMethodNotAllowed, "Method not allowed")
+		return
+	}
+
+	requestID := middleware.GetRequestID(r.Context())
+
+	// Extract certificate ID from path: /api/v1/certificates/{id}/export/pkcs12
+	id := extractCertIDFromExportPath(r.URL.Path)
+	if id == "" {
+		ErrorWithRequestID(w, http.StatusBadRequest, "Certificate ID is required", requestID)
+		return
+	}
+
+	// Parse optional password from request body (may be empty)
+	var req struct {
+		Password string `json:"password"`
+	}
+	// Body is optional — empty body means empty password
+	_ = parseJSONBody(r, &req)
+
+	pfxData, err := h.svc.ExportPKCS12(r.Context(), id, req.Password)
+	if err != nil {
+		if strings.Contains(err.Error(), "not found") {
+			ErrorWithRequestID(w, http.StatusNotFound, "Certificate not found", requestID)
+			return
+		}
+		ErrorWithRequestID(w, http.StatusInternalServerError, "Failed to export PKCS#12", requestID)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/x-pkcs12")
+	w.Header().Set("Content-Disposition", "attachment; filename=\"certificate.p12\"")
+	w.WriteHeader(http.StatusOK)
+	w.Write(pfxData)
+}
+
+// extractCertIDFromExportPath extracts the certificate ID from an export path.
+// Path format: /api/v1/certificates/{id}/export/pem or /api/v1/certificates/{id}/export/pkcs12
+func extractCertIDFromExportPath(path string) string {
+	prefix := "/api/v1/certificates/"
+	if !strings.HasPrefix(path, prefix) {
+		return ""
+	}
+	rest := strings.TrimPrefix(path, prefix)
+	// rest should be "{id}/export/pem" or "{id}/export/pkcs12"
+	parts := strings.Split(rest, "/")
+	if len(parts) < 3 || parts[1] != "export" {
+		return ""
+	}
+	return parts[0]
+}
+
+// parseJSONBody is a helper that decodes JSON from the request body.
+// Returns an error if the body is malformed, nil if body is empty.
+func parseJSONBody(r *http.Request, v interface{}) error {
+	if r.Body == nil {
+		return nil
+	}
+	return json.NewDecoder(r.Body).Decode(v)
+}