docs(marketing): track Cheaper Inference link clicks via ?utm_source=omniroute (#9258)

Adds the `?utm_source=omniroute` tracking parameter to every public-facing URL where Cheaper Inference is clickable.

- README: the two `<a href>` targets in the Open Source Friends table row (logo + "Get an API key" CTA)
- `gateways.ts`: the `website` field and the `apiHint` text

Not changed on purpose: `api.cheaperinference.com/*` endpoints (technical, not clicks), JSDoc mentions (descriptive text), and the `<sub>cheaperinference.com</sub>` label under the logo (plain text, not a link).

src/shared/constants/providers/apikey/gateways.ts

@@ -14,9 +14,9 @@ export const APIKEY_PROVIDERS_GATEWAYS = {
     icon: "savings",
     color: "#31f889",
     textIcon: "CI",
-    website: "https://cheaperinference.com",
+    website: "https://cheaperinference.com/?utm_source=omniroute",
     apiHint:
-      "Create an API key at https://cheaperinference.com (needs the `inference` scope), then paste the ir_live_… token here.",
+      "Create an API key at https://cheaperinference.com/?utm_source=omniroute (needs the `inference` scope), then paste the ir_live_… token here.",
     passthroughModels: true,
   },
   "charm-hyper": {