add missing configuration in BuildService function

[sgao640]

@tendant
@livefire2015

[livefire2015]

Could you use AI to post a summary in each PR?

[sgao640]

Purpose: Add environment-driven URL generation strategy support to the server config.

File changed: pkg/simplecontent/config/env.go.

New env vars:

• URL_STRATEGY (allowed: "content-based", "storage-delegated", "cdn")
• API_BASE_URL
• CDN_BASE_URL
• UPLOAD_BASE_URL

Implementation:

• Adds applyURLStrategyEnv which reads/validates those env vars and sets ServerConfig fields (URLStrategy, APIBaseURL, CDNBaseURL, UploadBaseURL).
• WithEnv now calls applyURLStrategyEnv during env parsing.

Validation: rejects unknown URL_STRATEGY values; other vars are applied if present.

No tests or additional validation (e.g., URL format or storage compatibility) included in this PR.