fix(downloader): apply url_base to download-client base URL (#369)

[SAY-5]

Summary

Every downloader client (qbittorrent, transmission, deluge, nzbget, sabnzbd) built its connection URL from host:port only. The url_base column is read from the DB into DownloadClient.URLBase but never interpolated into the URL, so operators running an *arr-compatible client behind a reverse-proxy subpath (e.g. Host 192.168.1.10:8080, URL Base /qbit) saw Bindery connect to http://192.168.1.10:8080/api/... instead of http://192.168.1.10:8080/qbit/api/....

Fix

• New internal/downloader/urlbase helper that normalises user input: trims whitespace, enforces a single leading slash, strips trailing slashes, and tolerates a pasted full URL by keeping only the path segment.
• Thread a urlBase parameter through each client's New constructor and apply the normalised prefix to baseURL / Transmission's RPC Path.
• Update every call site in internal/downloader/adapter.go and internal/importer/scanner.go to pass client.URLBase.
• Update all in-package test helpers so the signature changes stay compile-clean.
• Empty / whitespace url_base collapses to "", preserving the pre-Download client ignores url_base field — reverse proxy subpath not applied #369 URL shape for deployments without a proxy.

Fixes #369

Test plan

• go build ./...
• go test ./... — full suite green, including the existing per-client tests (deluge / nzbget / qbittorrent / sabnzbd / transmission) and the new internal/downloader/urlbase table-test
• Manually traced each New call site in adapter.go and scanner.go to confirm client.URLBase is wired in

[vavallee]

Clean implementation — dedicated urlbase package is the right call, and the paste-full-URL normalisation is a nice UX touch that the other approach missed. All checks look good on our side; approving to trigger CI.