Build every database's schema from embedded SQL files (closes #56)

The main database, each webhook's event database and each archive file
get their schema from SQL files under internal/db/migrations/, one
directory per kind. Opening a file runs, in one transaction, each file
that schema_migrations does not list. GORM stays for queries.
AutoMigrate is gone, tests included.

An event database holds only its own tables now, so the sweep that
removed target rows copied into it is deleted. The association guard
stays: without it a delivery write fails on the missing targets table.
The event totals row is inserted by events/001_schema.sql rather than
on every open.

Model: opus-5-5

internal/database/webhook_db_manager.go

@@ -14,6 +14,7 @@ import (
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/datadir"
+	"sneak.berlin/go/webhooker/internal/db/migrations"
 	"sneak.berlin/go/webhooker/internal/gormlog"
 )
 
@@ -343,6 +344,18 @@ func (m *WebhookDBManager) openDB(
 		)
 	}
 
+	err = migrations.Apply(
+		context.Background(), sqlDB, migrations.Events,
+	)
+	if err != nil {
+		_ = sqlDB.Close()
+
+		return nil, fmt.Errorf(
+			"migrating webhook database %s: %w",
+			webhookID, err,
+		)
+	}
+
 	db, err := gorm.Open(sqlite.Dialector{
 		Conn: sqlDB,
 	}, &gorm.Config{
@@ -370,39 +383,6 @@ func (m *WebhookDBManager) openDB(
 		)
 	}
 
-	err = purgeTargetRows(db, m.log, webhookID)
-	if err != nil {
-		_ = sqlDB.Close()
-
-		return nil, err
-	}
-
-	// Run migrations for event-tier models only
-	err = db.AutoMigrate(
-		&Event{}, &Delivery{}, &DeliveryResult{},
-		&EventTotals{}, &TargetTotals{}, &EntrypointTotals{},
-	)
-	if err != nil {
-		_ = sqlDB.Close()
-
-		return nil, fmt.Errorf(
-			"migrating webhook database %s: %w",
-			webhookID, err,
-		)
-	}
-
-	// A new database gets its row of event totals, all zero. Target
-	// totals rows are created by the first delivery to each target.
-	err = db.FirstOrCreate(&EventTotals{}).Error
-	if err != nil {
-		_ = sqlDB.Close()
-
-		return nil, fmt.Errorf(
-			"creating event totals for webhook database %s: %w",
-			webhookID, err,
-		)
-	}
-
 	m.log.Info(
 		"opened per-webhook database",
 		"webhook_id", webhookID,