docs: update CDN host example from media.mirrorstack.ai to cdn.mirrorstack.ai

[I-am-nothing]

Problem

SDK example comments reference https://media.mirrorstack.ai as the CDN host. Deployed reality (decided 2026-04-25) is one global CDN host: https://cdn.mirrorstack.ai. Update doc/example strings so SDK consumers see the right domain.

Scope

• storage/credential.go:18 — update CDNBase example comment from https://media.mirrorstack.ai to https://cdn.mirrorstack.ai
• storage/storage.go:74 — update cdnBase example reference to match
• examples/template/storage.go — if it references media.mirrorstack.ai, update
• docs/api-reference.md and README.md — search/replace any remaining media.mirrorstack.ai references
• No code/behavior change

Out-of-scope

• Per-module subdomain support (decided against; one global host)
• Storage interface changes

Acceptance criteria

• No occurrences of media.mirrorstack.ai remain in repo (verify with grep -r)
• Tests still pass (go test ./...)
• CHANGELOG entry under "Unreleased / Documentation"

Dependencies

None (purely documentation/example update)