feat: image output token billing, channel-mapped billing source, credits balance precheck

- Parse candidatesTokensDetails from Gemini API to separate image/text output tokens
- Add image_output_tokens and image_output_cost to usage_log (migration 089)
- Support per-image-token pricing via output_cost_per_image_token from model pricing data
- Channel pricing ImageOutputPrice override works in token billing mode
- Auto-fill image_output_price in channel pricing form from model defaults
- Add "channel_mapped" billing model source as new default (migration 088)
- Bills by model name after channel mapping, before account mapping
- Fix channel cache error TTL sign error (115s → 5s)
- Fix Update channel only invalidating new groups, not removed groups
- Fix frontend model_mapping clearing sending undefined instead of {}
- Credits balance precheck via shared AccountUsageService cache before injection
- Skip credits injection for accounts with insufficient balance
- Don't mark credits exhausted for "exhausted your capacity on this model" 429s

frontend/src/components/admin/channel/PricingEntryCard.vue

@@ -328,6 +328,7 @@ async function onModelsUpdate(newModels: string[]) {
         output_price: perTokenToMTok(result.output_price ?? null),
         cache_write_price: perTokenToMTok(result.cache_write_price ?? null),
         cache_read_price: perTokenToMTok(result.cache_read_price ?? null),
+        image_output_price: perTokenToMTok(result.image_output_price ?? null),
       })
     }
   } catch {