db: integrate golang-migrate for schema migrations

- internal/db/migrations/: five embedded SQL files containing
  the migration SQL previously held as Go string literals.
  Files follow the NNN_description.up.sql naming convention
  required by golang-migrate's iofs source.
- internal/db/migrate.go: rewritten to use
  github.com/golang-migrate/migrate/v4 with the
  database/sqlite driver (modernc.org/sqlite, pure Go) and
  source/iofs for compile-time embedded SQL.
  - newMigrate() opens a dedicated *sql.DB so m.Close() does
    not affect the caller's shared connection.
  - Migrate() includes a compatibility shim: reads the legacy
    schema_version table and calls m.Force(v) before m.Up()
    so existing databases are not re-migrated.
  - LatestSchemaVersion promoted from var to const.
- internal/db/db.go: added path field to DB struct; Open()
  translates ':memory:' to a named shared-cache URI
  (file:mcias_N?mode=memory&cache=shared) so the migration
  runner can open a second connection to the same in-memory
  database without sharing the handle that golang-migrate
  will close on teardown.
- go.mod: added golang-migrate/migrate/v4 v4.19.1 (direct).
All callers unchanged. All tests pass; golangci-lint clean.

internal/db/db.go

@@ -12,19 +12,36 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"sync/atomic"
 	"time"
 
 	_ "modernc.org/sqlite" // register the sqlite3 driver
 )
 
+// memCounter generates unique names for in-memory shared-cache databases.
+var memCounter atomic.Int64
+
 // DB wraps a *sql.DB with MCIAS-specific helpers.
 type DB struct {
 	sql *sql.DB
+	// path is the DSN used to open this database.  For in-memory databases
+	// (originally ":memory:") it is a unique shared-cache URI of the form
+	// file:mcias_N?mode=memory&cache=shared so that a second connection can be
+	// opened to the same in-memory database (needed by the migration runner).
+	path string
 }
 
 // Open opens (or creates) the SQLite database at path and configures it for
 // MCIAS use (WAL mode, foreign keys, busy timeout).
 func Open(path string) (*DB, error) {
+	// Translate bare ":memory:" to a named shared-cache in-memory URI.
+	// This allows the migration runner to open a second connection to the
+	// same in-memory database without sharing the *sql.DB handle (which
+	// would be closed by golang-migrate when the migrator is done).
+	if path == ":memory:" {
+		path = fmt.Sprintf("file:mcias_%d?mode=memory&cache=shared", memCounter.Add(1))
+	}
+
 	// The modernc.org/sqlite driver is registered as "sqlite".
 	sqlDB, err := sql.Open("sqlite", path)
 	if err != nil {
@@ -34,7 +51,7 @@ func Open(path string) (*DB, error) {
 	// Use a single connection for writes; reads can use the pool.
 	sqlDB.SetMaxOpenConns(1)
 
-	db := &DB{sql: sqlDB}
+	db := &DB{sql: sqlDB, path: path}
 	if err := db.configure(); err != nil {
 		_ = sqlDB.Close()
 		return nil, err