fix(caddy): add cache-control headers for static assets, service worker, and index.html

[Just-Insane]

Related to closed PR #548 (fix/media-error-handling), which was split into smaller focused PRs.

Description

Without explicit Cache-Control headers, browsers apply heuristic caching to all responses — including sw.js and index.html. This causes stale service workers and stale app shells to persist after a deploy. Three header rules added to the Caddyfile:

• /assets/* — public, max-age=31536000, immutable: content-hashed filenames change on every build, so these are safe to cache permanently.
• /sw.js — no-cache: must always be revalidated so the browser picks up SW updates promptly.
• /index.html — no-cache: must always be revalidated so the browser gets the latest asset hashes on load.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

The cache header rules were partially AI assisted. I verified the immutable directive is appropriate for content-hashed assets and that no-cache (revalidate-always, not no-store) is correct for sw.js and index.html.