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/conflicts.go

@@ -5,6 +5,7 @@ import (
 	wsync "bookhoard/internal/sync"
 	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"time"
 
@@ -83,7 +84,7 @@ func (h *ConflictHandler) GetConflictsData(c *echo.Context) ([]ConflictDetailRes
 		conflicts, err = h.db.ListSyncConflictsByUser(ctx, user.ID)
 	}
 
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return nil, 0, 0, err
 	}
 
@@ -150,7 +151,7 @@ func (h *ConflictHandler) GetConflict(c *echo.Context) error {
 	conflictUUID := pgtype.UUID{Bytes: [16]byte(conflictID), Valid: true}
 	conflict, err := h.db.GetSyncConflict(context.Background(), conflictUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return echo.NewHTTPError(http.StatusNotFound, "conflict not found")
 		}
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get conflict")
@@ -214,7 +215,7 @@ func (h *ConflictHandler) ResolveConflict(c *echo.Context) error {
 	conflictUUID := pgtype.UUID{Bytes: [16]byte(conflictID), Valid: true}
 	conflict, err := h.db.GetSyncConflict(context.Background(), conflictUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return echo.NewHTTPError(http.StatusNotFound, "conflict not found")
 		}
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get conflict")
@@ -292,7 +293,7 @@ func (h *ConflictHandler) applyProgressResolution(mediaItemID pgtype.UUID, userI
 		MediaItemID: mediaItemID,
 		UserID:      userID,
 	})
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return err
 	}
 
@@ -380,7 +381,7 @@ func (h *ConflictHandler) DeleteConflict(c *echo.Context) error {
 	conflictUUID := pgtype.UUID{Bytes: [16]byte(conflictID), Valid: true}
 	conflict, err := h.db.GetSyncConflict(context.Background(), conflictUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return echo.NewHTTPError(http.StatusNotFound, "conflict not found")
 		}
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get conflict")
@@ -641,7 +642,7 @@ func (h *ConflictHandler) applyResolution(mediaItemID pgtype.UUID, userID pgtype
 		MediaItemID: mediaItemID,
 		UserID:      userID,
 	})
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return err
 	}