chore: name the database in every migration log line

An operator reported migrations being very slow and sent a log with three
interleaved runs — and no way to tell one database migrated three times from
three databases migrated once. Each line now carries its target: the SQLite file
name, mysql:<database>, or baseline:memory for the in-memory pass that derives
the schema for a FRESH MySQL database (that one is fast and is not a real
database — in the report it sat between two slow runs and looked like a third).

A pass also announces how many migrations it will apply, so a run that applies
nothing is visibly distinct from one that does.

internal/db/db.go

@@ -6,6 +6,7 @@ import (
 	"embed"
 	"fmt"
 	"os"
+	"path/filepath"
 	"sort"
 	"strings"
 	"time"
@@ -161,7 +162,7 @@ func Open(path string) (*sql.DB, error) {
 		return nil, fmt.Errorf("ping sqlite: %w", err)
 	}
 	Dialect = "sqlite"
-	if err := migrate(conn, nil, path); err != nil {
+	if err := migrate(conn, nil, path, filepath.Base(path)); err != nil {
 		_ = conn.Close()
 		return nil, err
 	}
@@ -175,8 +176,8 @@ var LogSink func(format string, args ...any)
 
 // logMigration records a migration that has just been applied, and how long it
 // took — the only trace an operator has that a data-rewriting migration ran.
-func logMigration(name string, start time.Time) {
-	logf("db: migration %s applied in %s", name, time.Since(start).Round(time.Millisecond))
+func logMigration(label, name string, start time.Time) {
+	logf("db[%s]: migration %s applied in %s", label, name, time.Since(start).Round(time.Millisecond))
 }
 
 func logf(format string, args ...any) {
@@ -231,7 +232,11 @@ func backupBeforeRewrite(conn *sql.DB, dbPath, migration string) {
 // skipping those already applied. Intentionally minimal in-house system
 // (no external dependency). translate, when non-nil, rewrites each statement
 // for a non-SQLite backend (see mysqlDDL); nil means run the SQLite DDL as-is.
-func migrate(conn *sql.DB, translate func(string) string, dbPath string) error {
+// label names the database being migrated, for the log. Without it three
+// interleaved migration runs in one log were indistinguishable — an operator
+// reported "migrations are very slow" and the lines gave no way to tell one
+// database migrated three times from three databases migrated once.
+func migrate(conn *sql.DB, translate func(string) string, dbPath, label string) error {
 	// A non-nil translator means this is the MySQL connection (use the
 	// per-statement, FK-aware path); nil means a SQLite connection. This is
 	// determined by the caller's argument, NOT the global Dialect, so the
@@ -274,6 +279,16 @@ func migrate(conn *sql.DB, translate func(string) string, dbPath string) error {
 		return fmt.Errorf("read applied migrations: %w", err)
 	}
 
+	pending := 0
+	for _, name := range names {
+		if !applied[name] {
+			pending++
+		}
+	}
+	if pending > 0 {
+		logf("db[%s]: %d migration(s) to apply", label, pending)
+	}
+
 	for _, name := range names {
 		if applied[name] {
 			continue // already applied
@@ -307,7 +322,7 @@ func migrate(conn *sql.DB, translate func(string) string, dbPath string) error {
 			if _, err := conn.Exec(`INSERT INTO schema_migrations(name) VALUES(?)`, name); err != nil {
 				return fmt.Errorf("record migration %s: %w", name, err)
 			}
-			logMigration(name, start)
+			logMigration(label, name, start)
 			continue
 		}
 
@@ -327,7 +342,7 @@ func migrate(conn *sql.DB, translate func(string) string, dbPath string) error {
 		if err := tx.Commit(); err != nil {
 			return fmt.Errorf("commit migration %s: %w", name, err)
 		}
-		logMigration(name, start)
+		logMigration(label, name, start)
 	}
 	return nil
 }