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.

go.mod

@@ -4,10 +4,13 @@ go 1.26.0
 
 require (
 	github.com/golang-jwt/jwt/v5 v5.3.1
+	github.com/golang-migrate/migrate/v4 v4.19.1
 	github.com/google/uuid v1.6.0
 	github.com/pelletier/go-toml/v2 v2.2.4
-	golang.org/x/crypto v0.33.0
-	golang.org/x/term v0.29.0
+	golang.org/x/crypto v0.45.0
+	golang.org/x/term v0.37.0
+	google.golang.org/grpc v1.74.2
+	google.golang.org/protobuf v1.36.7
 	modernc.org/sqlite v1.46.1
 )
 
@@ -17,12 +20,10 @@ require (
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546 // indirect
-	golang.org/x/net v0.29.0 // indirect
+	golang.org/x/net v0.47.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect
-	golang.org/x/text v0.22.0 // indirect
-	google.golang.org/genproto/googleapis/rpc v0.0.0-20240903143218-8af14fe29dc1 // indirect
-	google.golang.org/grpc v1.68.0 // indirect
-	google.golang.org/protobuf v1.36.0 // indirect
+	golang.org/x/text v0.31.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20250818200422-3122310a409c // indirect
 	modernc.org/libc v1.67.6 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect