fix(game-data): degrade gracefully when items_game_cdn is unavailable

[marttinm]

Problem

`items_game_cdn.txt` was removed from SteamDatabase/GameTracking-CS2. The download URL in `lib/game_data.js` now returns 404:

```
https://raw.githubusercontent.com/SteamDatabase/GameTracking-CS2/master/game/csgo/pak01_dir/scripts/items/items_game_cdn.txt
```

Confirmed via `gh api search/code` against SteamDatabase/GameTracking-CS2: the file no longer exists at any path (0 results).

Because of that, `this.items_game_cdn` stays `false` forever, and the guard in `addAdditionalItemProperties()`:

```js
if (!this.items_game || !this.items_game_cdn || !this.csgo_english) return;
```

aborts enrichment for every single item, so the price service receives responses with no usable float/paint data ("Inspect service returned no usable float data").

Upstream status

Open, unmerged upstream PR csfloat#250 ("Changed service to steamapis") addresses the same root cause by swapping the dead GameTracking-CS2 URL for a paid third-party API (`api.steamapis.com/image/items/730`, requires an API key). That's the right call for consumers who need `imageurl`, but it adds an external paid dependency we don't need: nothing in our price_service (`floatInspectClient.js`) reads `imageurl` — only `floatvalue`, `paintseed`, and `paintindex` are consumed.

Fix

Instead of replacing the CDN source, make it optional:

• Split the guard so enrichment only requires `items_game` and `csgo_english` (which are still fetched successfully).
• Make the `imageurl` lookup conditional on `items_game_cdn` actually being loaded, instead of blocking the whole function.
• `items_game_cdn_url` fetch attempt is left in place (harmless if it keeps 404ing; free to revisit if GameTracking-CS2 restores/relocates the file).

No new external dependency, no API key required, smaller diff than upstream csfloat#250.

Test plan

No test harness in this repo. Manual smoke test (documented here, not committed):

```js
const GameData = require('./lib/game_data.js');
const gd = Object.create(GameData.prototype);
gd.items_game = { paint_kits: {...}, items: {...}, rarities:{}, qualities:{}, sticker_kits:{}, keychain_definitions:{} };
gd.items_game_cdn = false; // simulates the permanent 404 state
gd.csgo_english = new Proxy({...}, LanguageHandler);
gd.schema = { originNames: [] };
gd.addAdditionalItemProperties(iteminfo);
```

Result with `items_game_cdn = false`:

• Before fix: function returns immediately, `iteminfo` unchanged.
• After fix: `iteminfo` gets `min`, `max`, `weapon_type`, `item_name`, `full_item_name` populated correctly; `imageurl` is simply absent (as expected, since there's no CDN data to map it from). No crash.

Deploy plan (not executed by this PR)

Server compose builds this service from `../inspect` (sibling of `magicmarket_docker`):

1. Merge this PR to `master`.
2. On the server: `git pull` in the `inspect` checkout.
3. `docker compose build inspect`
4. `docker compose up -d inspect`