refactor(store): schema_version migration table + DeleteAlert FK fix

Replace the error-string-matching migration runner with a proper
schema_version table. Migrations are now numbered and recorded;
only unapplied versions run. Fresh databases seed at baseline
version (CREATE TABLE already includes all columns).

CREATE TABLE statements updated to include regions (sites) and
node_id (check_history) — previously only added via ALTER.

DeleteAlert now nulls sites.alert_id before deleting, preventing
dangling references that caused every incident to hit the error
path instead of alerting.

internal/store/dialect.go

@@ -5,10 +5,16 @@ import (
 	"strconv"
 )
 
+type Migration struct {
+	Version int
+	SQL     string
+}
+
 type Dialect interface {
 	DriverName() string
 	CreateTablesSQL() []string
-	MigrationsSQL() []string
+	Migrations() []Migration
+	BaselineVersion() int
 	BoolFalse() string
 	ResetSequenceOnEmpty(db *sql.DB, table string)
 	ImportWipe(tx *sql.Tx)