Add missing database migrations

Created by: jedmund

Summary

• Add migration for sourceImageUrl column on GardenItem — was added via raw SQL locally but never got a proper migration, breaking production
• Add migration for excerpt and syndicationText columns on Post — excerpt was dropped by a previous migration but re-added to the schema, syndicationText never had a migration at all

Both use IF NOT EXISTS so they're safe to run on databases that already have the columns.

Test plan

• Deploy and verify prisma migrate deploy runs without errors
• Garden CRUD works in production
• Post creation/editing works in production