feat: add pricing columns to stack_template + enrich webhook payload

[vsilent]

Step 0: Pricing persistence

• Add price (DOUBLE PRECISION), billing_cycle, currency columns to stack_template
• Accept plan_type, price, currency in CreateTemplateRequest and UpdateTemplateRequest
• Store pricing in create_draft() and update_metadata() DB functions
• Migration: 20260211100000_add_pricing_to_stack_template

Step 1: Fix vendor_name bug

• Webhook now sends template.creator_name instead of user ID string

Step 2: Enrich webhook payload

• Add long_description, tech_stack, creator_name, deploy_count, view_count, approved_at, required_plan_name fields to MarketplaceWebhookPayload
• Send actual price/billing_cycle/currency from DB instead of hardcoded None
• Both send_template_approved and send_template_updated updated

Note: sqlx offline cache (.sqlx/) needs regeneration after migration.
Run: cargo sqlx prepare -- --lib