fix: add missing velocity_24h and ple_delta_24h columns to campaigns table

The model includes Velocity24h and PleDelta24h fields but the database
was missing these columns, causing SQLSTATE 42703 errors when querying
campaigns.

Added idempotent migration to ensure both columns exist with proper
defaults (0) when the backend starts.

Related to deployment aa9ad5e - columns were added to model but not
migrated to dev database.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Jing-yilin

backend/internal/db/db.go

@@ -94,6 +94,31 @@ func Init(cfg *config.Config) error {
 		return fmt.Errorf("migrate campaigns pk: %w", err)
 	}
 
+	// Ensure velocity_24h and pledge_delta_24h columns exist (added in develop branch)
+	if err := DB.Exec(`
+		DO $$
+		BEGIN
+			-- Add velocity_24h column if it doesn't exist
+			IF NOT EXISTS (
+				SELECT 1 FROM information_schema.columns
+				WHERE table_name = 'campaigns' AND column_name = 'velocity_24h'
+			) THEN
+				ALTER TABLE campaigns ADD COLUMN velocity_24h DOUBLE PRECISION DEFAULT 0;
+			END IF;
+
+			-- Add pledge_delta_24h column if it doesn't exist (named ple_delta_24h in DB due to GORM naming)
+			IF NOT EXISTS (
+				SELECT 1 FROM information_schema.columns
+				WHERE table_name = 'campaigns' AND column_name = 'ple_delta_24h'
+			) THEN
+				ALTER TABLE campaigns ADD COLUMN ple_delta_24h DOUBLE PRECISION DEFAULT 0;
+			END IF;
+		END
+		$$;
+	`).Error; err != nil {
+		return fmt.Errorf("migrate velocity columns: %w", err)
+	}
+
 	log.Println("Database connected and migrated")
 	return nil
 }