test(storage): guard against duplicate goose migration version prefixes (#336)

* test(storage): guard against duplicate goose migration version prefixes

Statically scans embedded migration filenames for repeated numeric
version prefixes and fails with a clear message, catching the class
of bug from #333 (two PRs adding 0014_*.sql independently) before
goose.Up() would panic at runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(storage): dedupe migration versions by goose's parsed int64, not raw string

versionPrefix compared raw filename prefixes, so 014_x.sql and 0014_y.sql
were treated as distinct even though goose.NumericComponent parses both
to version 14 and panics on the collision. Use goose.NumericComponent
directly so the test enforces goose's actual collision rule.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(storage): trim PR-specific framing from migration version test comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/storage/sqlite/migrate_unique_version_test.go

@@ -0,0 +1,39 @@
+package sqlite
+
+import (
+	"testing"
+
+	"github.com/pressly/goose/v3"
+)
+
+// TestMigrationVersionsAreUnique scans the embedded migration filenames and
+// parses each version with goose.NumericComponent — the same function goose
+// itself uses — so prefixes that parse to the same int64 (e.g. "014" vs
+// "0014") are caught as a collision, not just identical strings. Catches the
+// conflict with a clear message instead of a goose panic at runtime.
+func TestMigrationVersionsAreUnique(t *testing.T) {
+	entries, err := migrationsFS.ReadDir("migrations")
+	if err != nil {
+		t.Fatalf("read migrations dir: %v", err)
+	}
+
+	seen := map[int64]string{} // parsed version -> filename
+	for _, e := range entries {
+		name := e.Name()
+		if e.IsDir() {
+			continue
+		}
+
+		version, err := goose.NumericComponent(name)
+		if err != nil {
+			t.Errorf("migration %q has no version goose can parse: %v", name, err)
+			continue
+		}
+
+		if other, dup := seen[version]; dup {
+			t.Errorf("duplicate migration version %d: %s vs %s", version, other, name)
+			continue
+		}
+		seen[version] = name
+	}
+}