refactor: remove unnecessary type conversions and handle ignored errors across codebase

Remove redundant type conversions that Go 1.26 makes unnecessary or that
were already no-ops:

- uuid.UUID(x.Bytes) → x.Bytes (uuid.UUID is [16]byte, same as pgtype UUID Bytes)
- pgtype.UUID{Bytes: [16]byte(u), Valid: true} → pgtype.UUID{Bytes: u, Valid: true}
- (*time.Time)(&x.Time) → &x.Time
- json.RawMessage(x) → x where x is already []byte
- []byte(stringVal) → stringVal where []byte is expected
- int()/int64()/byte() casts on values already of the target type
- Decompressor(fn) → fn (type is identical)

Handle previously ignored error returns:

- collections.go: check json.Unmarshal error in GetCollection
- conversion_service.go: check fileSize.Scan() error
- app_test.go: check app.Shutdown() error in benchmark

internal/handlers/refresh_token.go

@@ -27,7 +27,7 @@ func parseTokenUUID(tokenStr string) (pgtype.UUID, error) {
 	if err != nil {
 		return pgtype.UUID{}, err
 	}
-	return pgtype.UUID{Bytes: [16]byte(tokenUUID), Valid: true}, nil
+	return pgtype.UUID{Bytes: tokenUUID, Valid: true}, nil
 }
 
 type RefreshTokenResponse struct {
@@ -103,8 +103,8 @@ func (h *AuthHandler) CreateRefreshToken(userID uuid.UUID) (string, string, erro
 
 	expiresAt := time.Now().Add(refreshTokenExpiration)
 	_, err := h.db.CreateRefreshToken(context.Background(), database.CreateRefreshTokenParams{
-		UserID:    pgtype.UUID{Bytes: [16]byte(userID), Valid: true},
-		Token:     pgtype.UUID{Bytes: [16]byte(tokenUUID), Valid: true},
+		UserID:    pgtype.UUID{Bytes: userID, Valid: true},
+		Token:     pgtype.UUID{Bytes: tokenUUID, Valid: true},
 		ExpiresAt: pgtype.Timestamptz{Time: expiresAt, Valid: true},
 	})
 	if err != nil {