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/sync/queue_test.go

@@ -50,10 +50,10 @@ func TestSyncStatusConstants(t *testing.T) {
 }
 
 func TestPriorityConstants(t *testing.T) {
-	assert.Equal(t, int(1), PriorityUserInitiated)
-	assert.Equal(t, int(2), PriorityBookCompletion)
-	assert.Equal(t, int(3), PriorityCriticalNote)
-	assert.Equal(t, int(5), PriorityPageTurn)
-	assert.Equal(t, int(7), PriorityCheckpoint)
-	assert.Equal(t, int(10), PriorityBackgroundSync)
+	assert.Equal(t, 1, PriorityUserInitiated)
+	assert.Equal(t, 2, PriorityBookCompletion)
+	assert.Equal(t, 3, PriorityCriticalNote)
+	assert.Equal(t, 5, PriorityPageTurn)
+	assert.Equal(t, 7, PriorityCheckpoint)
+	assert.Equal(t, 10, PriorityBackgroundSync)
 }