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/services/dashboard_service.go

@@ -154,7 +154,7 @@ func (s *DashboardService) GetDashboardSections(
 		}
 
 		results = append(results, DashboardSection{
-			CollectionID:   uuid.UUID(coll.ID.Bytes),
+			CollectionID:   coll.ID.Bytes,
 			CollectionName: coll.Name,
 			Items:          items,
 			QueryType:      coll.QueryType.String,
@@ -182,7 +182,7 @@ func (s *DashboardService) GetDashboardSections(
 		}
 
 		results = append(results, DashboardSection{
-			CollectionID:   uuid.UUID(coll.ID.Bytes),
+			CollectionID:   coll.ID.Bytes,
 			CollectionName: coll.Name,
 			Items:          items,
 			QueryType:      coll.QueryType.String,