feat(handlers): add system-wide scan settings handler

Create new SystemSettingsHandler for managing system-wide scan settings:
- GetScanSettings: retrieve scan frequency and auto-scan status
- UpdateScanSettings: update scan settings (15-1440 minutes range)
- Admin-only access (no user-specific data)
- Key-value based storage instead of per-user settings

Replaces per-user scan settings with centralized system configuration.
This handler is used by /api/libraries/scan-settings endpoints.

internal/handlers/system_settings.go

@@ -0,0 +1,111 @@
+package handlers
+
+import (
+	"bookhoard/internal/database"
+	"net/http"
+	"strconv"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/labstack/echo/v4"
+)
+
+type SystemSettingsHandler struct {
+	db *database.Queries
+}
+
+func NewSystemSettingsHandler(db *database.Queries) *SystemSettingsHandler {
+	return &SystemSettingsHandler{
+		db: db,
+	}
+}
+
+type UpdateScanSettingsRequest struct {
+	ScanFrequencyMinutes int32 `json:"scan_frequency_minutes" validate:"required,min=15,max=1440"`
+	AutoScanEnabled      bool  `json:"auto_scan_enabled"`
+}
+
+type ScanSettingsResponse struct {
+	ScanFrequencyMinutes int32  `json:"scan_frequency_minutes"`
+	AutoScanEnabled      bool   `json:"auto_scan_enabled"`
+	Message              string `json:"message,omitempty"`
+}
+
+func (h *SystemSettingsHandler) UpdateScanSettings(c echo.Context) error {
+	var req UpdateScanSettingsRequest
+	if err := c.Bind(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
+	}
+	if err := c.Validate(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
+	}
+
+	scanFrequencyValue := strconv.FormatInt(int64(req.ScanFrequencyMinutes), 10)
+	autoScanValue := strconv.FormatBool(req.AutoScanEnabled)
+
+	err := h.db.UpdateSystemSetting(c.Request().Context(), database.UpdateSystemSettingParams{
+		SettingKey:   "scan_frequency_minutes",
+		SettingValue: scanFrequencyValue,
+	})
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return c.JSON(http.StatusNotFound, map[string]string{"error": "system setting not found"})
+		}
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	err = h.db.UpdateSystemSetting(c.Request().Context(), database.UpdateSystemSettingParams{
+		SettingKey:   "auto_scan_enabled",
+		SettingValue: autoScanValue,
+	})
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return c.JSON(http.StatusNotFound, map[string]string{"error": "system setting not found"})
+		}
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	return c.JSON(http.StatusOK, ScanSettingsResponse{
+		ScanFrequencyMinutes: req.ScanFrequencyMinutes,
+		AutoScanEnabled:      req.AutoScanEnabled,
+		Message:              "scan settings updated successfully",
+	})
+}
+
+func (h *SystemSettingsHandler) GetScanSettings(c echo.Context) error {
+	scanFrequencySetting, err := h.db.GetSystemSetting(c.Request().Context(), "scan_frequency_minutes")
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return c.JSON(http.StatusOK, ScanSettingsResponse{
+				ScanFrequencyMinutes: 60,
+				AutoScanEnabled:      true,
+			})
+		}
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	autoScanSetting, err := h.db.GetSystemSetting(c.Request().Context(), "auto_scan_enabled")
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return c.JSON(http.StatusOK, ScanSettingsResponse{
+				ScanFrequencyMinutes: 60,
+				AutoScanEnabled:      true,
+			})
+		}
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	scanFrequency, err := strconv.ParseInt(scanFrequencySetting, 10, 32)
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "invalid scan frequency setting"})
+	}
+
+	autoScanEnabled, err := strconv.ParseBool(autoScanSetting)
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "invalid auto scan setting"})
+	}
+
+	return c.JSON(http.StatusOK, ScanSettingsResponse{
+		ScanFrequencyMinutes: int32(scanFrequency),
+		AutoScanEnabled:      autoScanEnabled,
+	})
+}