Restore dynamic OG image generation

The og/ route was disabled during the initial setup. Currently Meta.svelte uses the cover image directly via getCoverUrl() instead of a purpose-built 1200x630 OG image. Social sharing previews are either missing or use an arbitrarily sized cover.

Current State

• No src/routes/og/ route exists
• Meta.svelte sets og:image to the raw cover image src (getCoverUrl(cover))
• OG image meta tags claim 1200x630 PNG but the actual image may be a different size and format

Tasks

• Create src/routes/og/[...path]/+server.js route that generates 1200x630 OG images
• Decide on generation approach (satori, puppeteer, canvas, or static template)
• Update Meta.svelte to point og:image at the generated OG endpoint instead of the raw cover
• Ensure OG images are prerendered at build time for static hosting