Add quantity, bundle price, and link fields to listings

Summary

• Add three columns to listings: quantity (int, default 1), bundle_price (int, nullable), and link (text, nullable). Migration 0004_unknown_kitty_pryde.sql.
• Admin create/edit expose the new fields with validation (quantity >= 1; bundle price only applied when quantity > 1; link must start with http(s)://).
• Public detail page renders "$X each", adds "N available" when quantity > 1, and "or $Y for all N" when a bundle price is set. "more info" blue underlined link renders below the description when the listing has one.

Test plan

• pnpm db:migrate applies 0004 cleanly on existing data (existing rows default to quantity=1, null bundle, null link)
• Create a new listing with quantity=3 and bundle price, confirm it renders "3 available" and "or $Y for all 3"
• Edit an existing listing, add a link starting with https://, confirm the "more info" link renders and opens in a new tab
• Submitting quantity=0 or a link without http(s):// returns the form error