perf(config): lower default fetch_interval_max from 60s to 5s (#875)

## Summary

- Lower the default `fetch_interval_max` from 60s to 5s to reduce job pickup latency for common single-runner deployments
- PR #819 optimized defaults for 200-runner fleets, regressing single-runner pickup time from ~2s to ~65s
- Most deployments use few runners; large fleets can still tune this value higher in their config

## Impact

| `fetch_interval_max` | 1 runner pickup | 200 runners idle |
| -------------------- | --------------- | ---------------- |
| 60s (previous)       | up to **65s**   | 3.3 req/s        |
| **5s (new default)** | up to **5s**    | 40 req/s         |

Closes https://gitea.com/gitea/act_runner/issues/869

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/875
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Co-committed-by: Bo-Yi Wu <appleboy.tw@gmail.com>

internal/pkg/config/config.example.yaml

@@ -35,7 +35,7 @@ runner:
   # The maximum interval for fetching the job from the Gitea instance.
   # The runner uses exponential backoff when idle, increasing the interval up to this maximum.
   # Set to 0 or same as fetch_interval to disable backoff.
-  fetch_interval_max: 60s
+  fetch_interval_max: 5s
   # The base interval for periodic log flush to the Gitea instance.
   # Logs may be sent earlier if the buffer reaches log_report_batch_size
   # or if log_report_max_latency expires after the first buffered row.

internal/pkg/config/config.go

@@ -155,7 +155,7 @@ func LoadDefault(file string) (*Config, error) {
 		cfg.Runner.FetchInterval = 2 * time.Second
 	}
 	if cfg.Runner.FetchIntervalMax <= 0 {
-		cfg.Runner.FetchIntervalMax = 60 * time.Second
+		cfg.Runner.FetchIntervalMax = 5 * time.Second
 	}
 	if cfg.Runner.LogReportInterval <= 0 {
 		cfg.Runner.LogReportInterval = 5 * time.Second