Unwind the hash worker pool instead of abandoning it (closes #6)

hashPhase returned the moment recordRun failed and left the pool
running: the feeder parked forever on a full jobs channel and every
worker on a full results channel. Until #4 landed the process exited
before that mattered; now that runScan returns an error and unwinds,
the goroutines are a real leak.

The pool is now an owned hashPool. Its context is derived from the
scan's, every blocking send in the feeder and the workers selects on
ctx.Done(), the feeder closes jobs on every path out so the workers'
range always terminates, and hashPhase defers pool.stop(), which
cancels and then drains results until the last goroutine has exited.
Draining is the half that matters: a worker already parked on a send
cannot observe the cancellation until a receiver frees it.

ctx comes from cmd.Context() and is threaded through runScan,
syncScan, both worker pools and the database layer as the first
parameter throughout, so graceful interrupt handling has a path to
hook into rather than a pool to rewrite.

The walk pool never leaked, because walkPhase always drains its
events to close, but it has the same unbounded-send shape and gets
the same treatment, together with a ctx.Err() guard after the walk: a
cancelled walk leaves a partial size census, and the update phase
would read every file it never reached as vanished and delete its
record.

Tests drive the scan entry point against a database whose insert
trigger aborts, with a fixture large enough that the failure lands
partway through the hash phase with more runs queued than either pool
channel can hold, and assert that the scan fails instead of hanging
and that runtime.NumGoroutine polls back to its pre-scan baseline.

db.go

@@ -96,7 +96,7 @@ func openDB(path string) (*sql.DB, error) {
 
 // openScanDatabase opens the database for the scan subcommand, creating
 // the file, its parent directory, and the schema as needed.
-func openScanDatabase(path string) (*sql.DB, error) {
+func openScanDatabase(ctx context.Context, path string) (*sql.DB, error) {
 	err := os.MkdirAll(filepath.Dir(path), dbDirPerm)
 	if err != nil {
 		return nil, fmt.Errorf("create database directory: %w", err)
@@ -107,7 +107,7 @@ func openScanDatabase(path string) (*sql.DB, error) {
 		return nil, err
 	}
 
-	err = initSchema(db)
+	err = initSchema(ctx, db)
 	if err != nil {
 		_ = db.Close()
 
@@ -120,7 +120,9 @@ func openScanDatabase(path string) (*sql.DB, error) {
 // openReportDatabase opens an existing database for the report and
 // trees subcommands. A missing database file is an error directing the
 // user to run scan first; the schema version must match exactly.
-func openReportDatabase(path string) (*sql.DB, error) {
+func openReportDatabase(ctx context.Context,
+	path string,
+) (*sql.DB, error) {
 	_, err := os.Stat(path)
 	if errors.Is(err, fs.ErrNotExist) {
 		return nil, fmt.Errorf("%s: %w", path, errNoDatabase)
@@ -135,7 +137,7 @@ func openReportDatabase(path string) (*sql.DB, error) {
 		return nil, err
 	}
 
-	v, err := userVersion(db)
+	v, err := userVersion(ctx, db)
 	if err != nil {
 		_ = db.Close()
 
@@ -154,15 +156,15 @@ func openReportDatabase(path string) (*sql.DB, error) {
 
 // initSchema creates the schema on a fresh database and verifies the
 // schema version on an existing one.
-func initSchema(db *sql.DB) error {
-	v, err := userVersion(db)
+func initSchema(ctx context.Context, db *sql.DB) error {
+	v, err := userVersion(ctx, db)
 	if err != nil {
 		return err
 	}
 
 	switch v {
 	case 0:
-		return createSchema(db)
+		return createSchema(ctx, db)
 	case schemaVersion:
 		return nil
 	default:
@@ -173,9 +175,7 @@ func initSchema(db *sql.DB) error {
 
 // createSchema applies the schema to a fresh database and stamps the
 // schema version.
-func createSchema(db *sql.DB) error {
-	ctx := context.Background()
-
+func createSchema(ctx context.Context, db *sql.DB) error {
 	_, err := db.ExecContext(ctx, createTableSQL)
 	if err != nil {
 		return fmt.Errorf("create schema: %w", err)
@@ -191,11 +191,10 @@ func createSchema(db *sql.DB) error {
 }
 
 // userVersion reads the database's PRAGMA user_version.
-func userVersion(db *sql.DB) (int, error) {
+func userVersion(ctx context.Context, db *sql.DB) (int, error) {
 	var v int
 
-	err := db.QueryRowContext(context.Background(),
-		"PRAGMA user_version").Scan(&v)
+	err := db.QueryRowContext(ctx, "PRAGMA user_version").Scan(&v)
 	if err != nil {
 		return 0, fmt.Errorf("read schema version: %w", err)
 	}
@@ -204,8 +203,8 @@ func userVersion(db *sql.DB) (int, error) {
 }
 
 // loadFileRows reads every record from the files table.
-func loadFileRows(db *sql.DB) ([]scanRec, error) {
-	rows, err := db.QueryContext(context.Background(),
+func loadFileRows(ctx context.Context, db *sql.DB) ([]scanRec, error) {
+	rows, err := db.QueryContext(ctx,
 		"SELECT path, size, mtime, head, tail FROM files")
 	if err != nil {
 		return nil, fmt.Errorf("read records: %w", err)
@@ -242,10 +241,10 @@ func loadFileRows(db *sql.DB) ([]scanRec, error) {
 // it carries hashes to fn. Scan change detection needs no hash
 // values, and skipping the hash columns keeps the scan's in-memory
 // index small on multi-million-file databases.
-func loadFileMeta(db *sql.DB,
+func loadFileMeta(ctx context.Context, db *sql.DB,
 	fn func(path string, size, mtime int64, hashed bool),
 ) error {
-	rows, err := db.QueryContext(context.Background(),
+	rows, err := db.QueryContext(ctx,
 		"SELECT path, size, mtime, head <> '' FROM files")
 	if err != nil {
 		return fmt.Errorf("read records: %w", err)
@@ -286,18 +285,18 @@ const updateBatchSize = 10000
 // applyChanges writes one scan's database changes — upserts for new and
 // changed files, deletes for vanished ones — in batched transactions.
 // Progress is rendered on prog (one increment per change).
-func applyChanges(db *sql.DB, upserts []scanRec, deletes []string,
-	prog *progress,
+func applyChanges(ctx context.Context, db *sql.DB, upserts []scanRec,
+	deletes []string, prog *progress,
 ) error {
 	for batch := range slices.Chunk(upserts, updateBatchSize) {
-		err := applyBatch(db, batch, nil, prog)
+		err := applyBatch(ctx, db, batch, nil, prog)
 		if err != nil {
 			return err
 		}
 	}
 
 	for batch := range slices.Chunk(deletes, updateBatchSize) {
-		err := applyBatch(db, nil, batch, prog)
+		err := applyBatch(ctx, db, nil, batch, prog)
 		if err != nil {
 			return err
 		}
@@ -308,11 +307,9 @@ func applyChanges(db *sql.DB, upserts []scanRec, deletes []string,
 
 // applyBatch commits one batch of upserts and deletes in a single
 // transaction.
-func applyBatch(db *sql.DB, upserts []scanRec, deletes []string,
-	prog *progress,
+func applyBatch(ctx context.Context, db *sql.DB, upserts []scanRec,
+	deletes []string, prog *progress,
 ) error {
-	ctx := context.Background()
-
 	tx, err := db.BeginTx(ctx, nil)
 	if err != nil {
 		return fmt.Errorf("begin transaction: %w", err)