Store only the relative path in CardPrinting.imageUrl

[jbourdin]

Context

Currently `CardPrinting.imageUrl` stores the full CDN URL:
```
https://assets.tcgdex.net/en/sv/sv02/001/high.webp
```

With the local TCGdex model (#314), `TcgdexCard::getImageUrl()` already computes this from the entity hierarchy with configurable resolution and format. For cards sourced from the local DB, the URL is fully derivable.

Current Decision

Deferred. `CardPrinting.imageUrl` stays as a full URL for now because:

1. Multiple sources — the URL may come from TCGdex CDN, pokemontcg.io fallback, or static overrides. These have different base URLs and path conventions, so a single relative path format doesn't cover all cases.
2. Fallback chain (Image download fallback chain with URL update on success #316) — when a CDN URL returns 404, the resolved working URL is written back to `CardPrinting.imageUrl`. This URL may be from a different CDN entirely.
3. Existing CardPrinting rows — already store full URLs from the API. Migration would need to detect the source and strip accordingly.

Future Approach

Once #316 (fallback chain) is implemented and stabilized, we could:

1. Normalize all URLs to a `{source}:{path}` format (e.g. `tcgdex:sv/sv02/001`, `pokemontcgio:sv01/1`)
2. A URL builder resolves the full URL at render time with resolution/format params
3. This would save ~100 bytes per row across 23k+ cards

Dependencies

• After Local TCGdex card cache + DeckCard normalization #314 PR2 (card model refactoring)
• After Image download fallback chain with URL update on success #316 (image fallback chain — stabilizes the URL sources)