Extract `HostWhitelist`, `NewHostWhitelist`, `IsWhitelisted`, `IsEmpty`, and `Count` from `internal/imgcache/` into the new `internal/whitelist/` package.
The whitelist package is completely self-contained, depending only on `net/url` and `strings` from the standard library. No circular imports introduced.
**Changes:**
- Moved `whitelist.go` → `internal/whitelist/whitelist.go` (added package comment)
- Moved `whitelist_test.go` → `internal/whitelist/whitelist_test.go` (adapted to external test style)
- Updated `internal/imgcache/service.go` to import from `sneak.berlin/go/pixa/internal/whitelist`
`docker build .` passes (lint, tests, build).
Part of [issue #39](#39)
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #41
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>
