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/reader.go

@@ -4,6 +4,7 @@ import (
 	"bookhoard/internal/database"
 	"bookhoard/internal/services"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -63,7 +64,7 @@ func (h *ReaderHandler) ShowReader(c *echo.Context) error {
 	// Get media item
 	mediaItem, err := h.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: parsedUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "Media item not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "Failed to fetch media item"})
@@ -108,7 +109,7 @@ func (h *ReaderHandler) ShowReader(c *echo.Context) error {
 		MediaItemID: pgtype.UUID{Bytes: parsedUUID, Valid: true},
 		UserID:      userData.ID,
 	})
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		progress = database.ReadingProgress{}
 	}
 
@@ -316,7 +317,7 @@ func (h *ReaderHandler) GetReadingSpeed(c *echo.Context) error {
 	})
 
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			// Return zero values if no reading has occurred
 			return c.JSON(http.StatusOK, map[string]interface{}{
 				"words_per_minute":      0,
@@ -597,7 +598,7 @@ func (h *ReaderHandler) ParseEbook(c *echo.Context) error {
 	// Fetch media item
 	mediaItem, err := h.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: parsedUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(404, map[string]string{"error": "Media item not found"})
 		}
 		return c.JSON(500, map[string]string{"error": "Failed to fetch media item"})