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/refresh_token.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"bookhoard/internal/database"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
@@ -52,7 +53,7 @@ func (h *AuthHandler) RefreshAccessToken(c *echo.Context) error {
 
 	tokenInfo, err := h.db.GetRefreshToken(c.Request().Context(), tokenUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusUnauthorized, map[string]string{"error": "invalid or expired refresh token"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to validate refresh token"})
@@ -88,7 +89,7 @@ func (h *AuthHandler) Logout(c *echo.Context) error {
 	}
 
 	err = h.db.RevokeRefreshToken(c.Request().Context(), tokenUUID)
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to revoke refresh token"})
 	}