fix: implement migration tracking and make 0002 idempotent

- Add schema_migrations table to track applied migrations
- Migrate() now checks if a migration was already applied before running
- 0002 rewritten to use SAVEPOINT+ROLLBACK per column, making it safe
  to run even if some columns were partially added previously
- Each migration runs in its own transaction; INSERT into schema_migrations
  only happens if the SQL executes without error
- Bump VERSION 0.1.8 -> 0.1.9

Makefile

@@ -1,5 +1,5 @@
 BINARY=nasctl
-VERSION?=0.1.8
+VERSION?=0.1.9
 GO?=go
 LDFLAGS=-s -w -X github.com/darroyo/nasctl/internal/web.Version=$(VERSION) -X github.com/darroyo/nasctl/internal/web.Commit=$(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
 BUILD_FLAGS=CGO_ENABLED=0