Add cache validators to embedded static assets

[LarsLaskowski]

📖 Description

Files served from embed.FS carry a zero ModTime, so http.FileServerFS (internal/web/embed.go) emitted no Last-Modified/ETag, and no Cache-Control header was set — every dashboard page load refetched all static assets, and no intermediary could revalidate a cached copy against anything.

web.Handler now takes the build-time version string (already available as main.version, and echoed by GET /api/v1/config) and wraps the file server to set Cache-Control: no-cache plus a version-derived Etag on every response before handing off to http.FileServerFS. Go's stdlib conditional-request handling (checkPreconditions in net/http/fs.go) picks up an Etag set on the response writer before ServeContent runs, so this is enough for the file server to answer a matching If-None-Match with 304 Not Modified — no extra logic needed. After an upgrade changes version, the ETag changes too, so clients refetch immediately instead of serving stale JS.

This is an internal API change (web.Handler() → web.Handler(version string)); the only caller is cmd/pimonitor/main.go, updated to pass version. No REST API shape changes.

🎫 Issues

Closes #71

👩‍💻 Reviewer Notes

Worth double-checking the Etag/Cache-Control behavior directly against the running binary if you want an extra sanity check, e.g.:

curl -i http://localhost:8080/ | grep -i -e etag -e cache-control
curl -i -H 'If-None-Match: "<same-version>"' http://localhost:8080/   # expect 304

📑 Test Plan

Added to internal/web/embed_test.go:

• TestHandler_SetsCacheHeaders — asserts Cache-Control: no-cache and the version-derived Etag are present.
• TestHandler_RevalidatesOnMatchingETag — a request with a matching If-None-Match gets 304 Not Modified.
• TestHandler_RefetchesOnVersionChange — a stale If-None-Match (from a different version) still gets 200 OK.

Existing Handler(...) call sites in embed_test.go updated to pass a version string.

go build ./..., go vet ./..., go test ./... -race -cover, and golangci-lint run all pass locally.

✅ Checklist

General

• I have added/updated tests for my changes (go test ./... -race -cover passes locally).
• go vet ./... and golangci-lint run are clean.
• I have tested my changes.
• I have read the CONTRIBUTING documentation and followed the project's code style guidelines.
• I have updated ARCHITECTURE.md if this changes a documented design decision.

REST API / configuration / packaging

Not applicable — no REST API, configuration, or packaging changes.

⏭ Next Steps

None.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
85.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud