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/images.rs

@@ -247,6 +247,7 @@ pub async fn generate(app: &App, req: GenerateRequest) -> Result<Vec<ImageRecord
         cost,
         error: false,
         user_id: None,
+        allowance: None,
     });
     Ok(records)
 }