fix: resolve issues #20, #21, #22, #23, #24

[calmcacil]

Summary

Batch fix for 5 open issues and one follow-up (issue #8 closed as superseded).

Changes

Issue #20 — API tokens in query strings

• Jellyfin: moved api_key from URL query param to X-API-Key header
• Plex: documented as known API limitation (requires token in query string)

Issue #21 — Error messages to stdout

• Replaced all fmt.Printf error/diagnostic calls in internal/requests/requests.go with fmt.Fprintf(os.Stderr, ...)

Issue #22 — HTTP response handling boilerplate

• Added DoRequest, DoJSON, DoXML methods to internal/http/client.go
• Refactored fetchers in internal/streams/, internal/calendar/, internal/feed/ to use shared helpers
• Each fetcher went from ~15 lines of boilerplate to ~3

Issue #23 — Inconsistent CLI validation

• Added -server enum validation to media-streams
• Added -days/-days-past/-filter validation to media-calendar
• Added -url non-empty check to media-requests
• Added -events/ -poll validation to arr-feed
• All tools now call config.Validate() at startup
• Fixed arr-feed using println instead of stderr

Issue #24 — Missing godoc comments

• Added doc comments to all exported types, constants, and functions across internal/http, internal/config, internal/streams, internal/calendar, internal/requests, internal/feed

Issue #8 — Closed as superseded by config overhaul (PRs #18, #19)

Files changed: 13

Verification

• go build ./... — passes
• go test ./... — all tests pass
• go vet ./... — passes

Closes #20, #21, #22, #23, #24