feat(scanner): POST /api/scanner/cancel/:jobId (admin)

Cancels a pending or running scan job. Responses: 200 with the job id
and status on acceptance, 404 for unknown jobs, 409 for jobs already
in a terminal state (completed/failed/cancelled).

Additive route in the existing admin scanner group; no contract pins
affected.

internal/handlers/scanner.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"bookhoard/internal/services"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -209,6 +210,28 @@ func (h *Handler) GetScanSummary(c *echo.Context) error {
 	})
 }
 
+// CancelScan handles POST /api/scanner/cancel/:jobId (admin): requests
+// cancellation of a pending or running scan job. Cancellation is
+// cooperative — the file currently being processed finishes, the walk
+// stops at its next context check, and the archive pass is skipped so
+// unvisited files are never treated as missing.
+func (h *Handler) CancelScan(c *echo.Context) error {
+	jobID := c.Param("jobId")
+
+	if err := h.worker.CancelJob(jobID); err != nil {
+		if errors.Is(err, services.ErrJobNotFound) {
+			return c.JSON(http.StatusNotFound, map[string]string{"error": "job not found"})
+		}
+		return c.JSON(http.StatusConflict, map[string]string{"error": err.Error()})
+	}
+
+	return c.JSON(http.StatusOK, map[string]interface{}{
+		"message": "scan cancellation requested",
+		"job_id":  jobID,
+		"status":  "cancelled",
+	})
+}
+
 // StartWatchModeForLibrary starts watching a specific library's folders
 func (h *Handler) StartWatchModeForLibrary(ctx context.Context, libraryID pgtype.UUID, adminID pgtype.UUID) error {
 	h.mu.Lock()

internal/router/scanner.go

@@ -17,6 +17,7 @@ func registerScannerRoutes(protected, admin *echo.Group, h *handlers.Handler) {
 	admin.POST("/scanner/scan", h.ScanLibrary)
 	admin.POST("/scanner/start", h.StartScanner)
 	admin.POST("/scanner/stop", h.StopScanner)
+	admin.POST("/scanner/cancel/:jobId", h.CancelScan)
 	admin.GET("/scanner/status/:jobId", h.GetScanStatus)
 	admin.GET("/scanner/active", h.GetActiveScans)