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/database/database.go

@@ -18,6 +18,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/banner"
 	"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"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
@@ -249,12 +250,21 @@ func (d *Database) connectTo(dataDir string) error {
 		return fmt.Errorf("migrating %s: %w", dbPath, err)
 	}
 
+	err = parseModels(
+		db, &Setting{}, &User{}, &APIKey{},
+		&Webhook{}, &Entrypoint{}, &Target{},
+	)
+	if err != nil {
+		return fmt.Errorf("connecting to %s: %w", dbPath, err)
+	}
+
 	return nil
 }
 
+// migrate applies the main database's SQL files. See
+// internal/db/migrations.
 func (d *Database) migrate() error {
-	// Run GORM auto-migrations
-	err := d.Migrate()
+	err := migrations.Apply(d.db, migrations.Main)
 	if err != nil {
 		d.log.Error(
 			"failed to run database migrations",
@@ -269,6 +279,21 @@ func (d *Database) migrate() error {
 	return nil
 }
 
+// parseModels has GORM parse each model for db before the handle is
+// shared. GORM parses a model the first time a handle uses it, and two
+// goroutines doing that at once for models that refer to each other,
+// as Webhook and Target do, race inside GORM.
+func parseModels(db *gorm.DB, models ...any) error {
+	for _, model := range models {
+		err := (&gorm.Statement{DB: db}).Parse(model)
+		if err != nil {
+			return fmt.Errorf("parsing model %T: %w", model, err)
+		}
+	}
+
+	return nil
+}
+
 // ensureAdminUser creates the bootstrap admin account when the
 // deployment has no users at all.
 func (d *Database) ensureAdminUser() error {