Remediate all lint findings under the canonical golangci-lint config

Fix every finding surfaced by the canonical .golangci.yml with
golangci-lint v2.12.2 (refs #61), behavior-preserving throughout:

- err113: dynamic errors replaced with package-level sentinels and %w
  wrapping; direct comparisons converted to errors.Is
- goprintffuncname: printf-style helpers renamed with an f suffix
  (ui.Writer message methods, cli.ReportErrorf, database.Fatalf,
  vaultik stdoutf) and all call sites updated
- revive: stuttering type names renamed (blob.Handler, blob.WithReader,
  blob.ChunkPosition, storage.URL, storage.Info), doc comments added,
  unused parameters blanked, package comments added
- contextcheck/noctx: ctx threaded through blob.Packer
  (AddChunk/Flush/FinalizeBlob/PackChunks) and scanner call sites;
  context-aware exec and sql variants used
- funlen/cyclop/gocognit/nestif/dupl: oversized or duplicated
  functions split into focused helpers across production and test code
- paralleltest/tparallel/thelper/usetesting/testpackage: tests
  parallelized where safe (global log.Initialize kept in the serial
  phase), helpers marked, t.TempDir adopted, external test packages
  where only exported API is used
- gosec: integer conversions clamped or justified, header timeouts
  added, remaining findings suppressed with per-site justifications
- mnd/goconst/lll/wsl_v5/nlreturn/noinlineerr/errcheck and other
  mechanical findings fixed directly

Remove the deprecated log.LogOptions alias (callers migrated to
log.Options). make check is green.

internal/database/helpers_internal_test.go

@@ -0,0 +1,81 @@
+package database
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"sneak.berlin/go/vaultik/internal/types"
+)
+
+// Common fixture values shared by the internal repository tests.
+const (
+	internalTestHost       = "test-host"
+	internalTestSnapshotID = "test-snapshot"
+	internalTestFilePath   = "/test.txt"
+	internalTestFile1      = "/file1.txt"
+	internalTestFile2      = "/file2.txt"
+
+	// countFilesQuery counts the rows of the files table.
+	countFilesQuery = "SELECT COUNT(*) FROM files"
+)
+
+// mustCreateFileRow inserts the file row, failing the test on error.
+func mustCreateFileRow(t *testing.T, repos *Repositories, file *File) {
+	t.Helper()
+
+	err := repos.Files.Create(context.Background(), nil, file)
+	if err != nil {
+		t.Fatalf("failed to create file %s: %v", file.Path, err)
+	}
+}
+
+// mustAddFileToSnapshot associates a file with a snapshot, failing the
+// test on error.
+func mustAddFileToSnapshot(
+	t *testing.T, repos *Repositories, snapshotID string, fileID types.FileID,
+) {
+	t.Helper()
+
+	err := repos.Snapshots.AddFileByID(context.Background(), nil, snapshotID, fileID)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+// setupTestDB creates an on-disk test database in a per-test temp
+// directory and returns it along with a cleanup func that closes it.
+func setupTestDB(t *testing.T) (*DB, func()) {
+	t.Helper()
+
+	ctx := context.Background()
+	dbPath := filepath.Join(t.TempDir(), "test.db")
+
+	db, err := New(ctx, dbPath)
+	if err != nil {
+		t.Fatalf("failed to create database: %v", err)
+	}
+
+	cleanup := func() {
+		err := db.Close()
+		if err != nil {
+			t.Errorf("failed to close database: %v", err)
+		}
+	}
+
+	return db, cleanup
+}
+
+// countRow runs a single-integer COUNT-style query and returns the value.
+func countRow(t *testing.T, db *DB, query string, args ...any) int {
+	t.Helper()
+
+	var count int
+
+	err := db.conn.QueryRowContext(context.Background(), query, args...).Scan(&count)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return count
+}