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.

AutoMigrate also had GORM parse every model before a handle was shared.
That is now done on its own: GORM races when two goroutines first use
models that refer to each other.

Model: opus-5-5

internal/delivery/terminal_state_test.go

@@ -312,13 +312,9 @@ func TestFailMissingTarget_WritesNoTargetRow(
 		database.DeliveryStatusFailed,
 	)
 
-	var configs []string
-
-	require.NoError(t, s.WebhookDB.
-		Table("targets").
-		Pluck("config", &configs).Error)
-
-	assert.Empty(t, configs,
+	// The per-webhook database has no targets table, so writing a
+	// target row there would have failed the status write above.
+	assert.False(t, s.WebhookDB.Migrator().HasTable("targets"),
 		"the deleted-target terminal path wrote a target row "+
 			"into the per-webhook event database",
 	)