opencode prices and Go limits, settings out of .env

- Fetch prices for opencode (Zen and Go) incl. limits, image input, reasoning,
  long-context tiers and peak-hour/date factors; costs apply them per request
- Record the cost opencode reports per response; track Go allowance in 5h/7d/month
  windows against a provider plan limit (budget table, status overlay)
- Settings → Config file: move values of ${NAME} references into the config
- Examples, presets and forms keep keys in the config instead of .env

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/cli_api.rs

@@ -175,6 +175,7 @@ pub async fn llm_stream(Extension(user): Extension<User>, State(app): State<AppS
             cost: model.cost(&usage),
             error,
             user_id: Some(user.id.clone()),
+            allowance: usage.allowance,
         });
     });
     let stream = futures::stream::unfold(out_rx, |mut rx| async move { rx.recv().await.map(|ev| (ev, rx)) });