refactor: use errors.Is()/errors.AsType() for error comparison and rename shadowed variables

Replace direct error equality checks (err == pgx.ErrNoRows, err != http.ErrServerClosed)
with the idiomatic errors.Is() function throughout handlers, services, middleware, and app
startup. This correctly handles wrapped error chains.

Also replace a raw type assertion (*HTTPError) with errors.AsType[*HTTPError]() in the
error handler middleware for consistency.

Additionally, rename shadowed variables for clarity:
- sidecar.go: config -> sidecarConfig, systemConfig (shadowed package-level vars)
- media_scanner.go: uuid -> uuidString (shadowed the uuid package import)

internal/handlers/analytics.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"bookhoard/internal/database"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
@@ -98,7 +99,7 @@ func (h *AnalyticsHandler) GetReadingStats(c *echo.Context) error {
 		CreatedAt_2: pgtype.Timestamptz{Time: endTime, Valid: true},
 	})
 
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get reading history")
 	}
 
@@ -211,7 +212,7 @@ func (h *AnalyticsHandler) GetDeviceUsage(c *echo.Context) error {
 	ctx := context.Background()
 
 	usage, err := h.db.GetUserDeviceUsage(ctx, user.ID)
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get device usage")
 	}
 
@@ -263,7 +264,7 @@ func (h *AnalyticsHandler) GetPopularBooks(c *echo.Context) error {
 		Limit:  limitInt,
 	})
 
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get popular books")
 	}