Fix: create the fiddle table with explicit DDL, not AutoMigrate

GORM's migrator crashes with "insufficient arguments" when it introspects
this composite-primary-key table if the table already exists — which is
exactly what happened on the first production rollout (my local end-to-end
test had created the table moments earlier), crashlooping the backend until
a rollout undo restored service.

CREATE TABLE IF NOT EXISTS is idempotent and has no such moods. A failure
now degrades to fiddles-off like every other storage failure, instead of
taking /transform down with it. Verified against all three paths: existing
table, fresh database, and restart over a self-created table.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KZntQUfw463NW4ftgSjWw5

backend/src/fiddle.go

@@ -76,8 +76,22 @@ func registerFiddleRoutes(r *gin.Engine, db *gorm.DB) {
 		return
 	}
 
-	if err := db.AutoMigrate(&Fiddle{}); err != nil {
-		log.Fatalf("fiddle migrate: %v", err)
+	// Explicit, idempotent DDL instead of GORM's AutoMigrate: the migrator
+	// chokes on this composite primary key when the table already exists
+	// ("insufficient arguments" on its introspection query), which crashlooped
+	// the first deployment. CREATE IF NOT EXISTS has no such moods.
+	if err := db.Exec(`CREATE TABLE IF NOT EXISTS fiddles (
+		id varchar(12) NOT NULL,
+		revision integer NOT NULL,
+		payload text NOT NULL,
+		created_at timestamptz NOT NULL DEFAULT now(),
+		PRIMARY KEY (id, revision)
+	)`).Error; err != nil {
+		// Same degradation contract as a failed connection: fiddles off,
+		// transforms unaffected.
+		log.Printf("fiddle table create failed, fiddle storage disabled: %v", err)
+		registerFiddleRoutes(r, nil)
+		return
 	}
 
 	r.POST("/fiddle", func(c *gin.Context) {