Fetches refuse internal addresses unless fetch_internal lists them (#320)

ipx's clients had no destination policy and followed redirects
anywhere. Adding a feed returned the status or first line of whatever
answered at the address; an enclosure declared as audio was downloaded
whole from any URL and served back through /media; /api/art fetched
any image a feed named. Feeds come from publishers and anyone signed
in, so loopback, the Docker network and the LAN were all reachable.

ipx-feed's guard is a connector layer: after the name resolves and the
connection opens, before the request is written, it looks at the
peer's address and refuses anything off the public internet unless
config.toml's [general] fetch_internal lists it. Being on the
connection, it covers every request on ctx.client and feed_client,
each redirect hop, IP literals (which never reach a DNS resolver) and
a name that resolves differently the second time. reqwest's rustls
connection reports the peer address; a test run by hand checks HTTPS
gets through, since were it lost every https fetch would be refused.

The directory is on the server's own network by design
(http://ipx-directory:8100), so it moves to local_client, unguarded,
set by an admin. The browser suite and load tests serve their feeds on
loopback and list 127.0.0.1.

Not covered: librqbit's own connections (trackers, peers), which do
not go through these clients.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/configuration.md

@@ -46,10 +46,11 @@ max_age_days      = 30              # 0 = keep forever
 max_new_per_check = 3               # per feed, per scan. 0 = unlimited
 media_types       = ["audio", "video"]
 art_cache_mb      = 500             # artwork kept on disk. 0 = none
+fetch_internal    = []              # internal addresses fetches may reach, e.g. ["192.168.1.0/24"]
 ```
 
 `schedule`, `max_total_gb`, `max_age_days`, `max_new_per_check`, `media_types` and `art_cache_mb` move into the
-database as described above; `download_dir`, `socket` and `organize` stay in config.toml.
+database as described above; `download_dir`, `socket`, `organize` and `fetch_internal` stay in config.toml.
 
 * **`schedule`** — how often feeds are re-checked. A feed's own `<ttl>` still wins when it asks to
   be polled *less* often, and a per-feed `schedule` overrides both. Without a schedule of its own,
@@ -77,6 +78,12 @@ database as described above; `download_dir`, `socket` and `organize` stay in con
 * **`media_types`** — top-level MIME types taken automatically. Anything else is still listed and
   can be fetched by hand; blog feeds put each article's header image in an `<enclosure>`, and
   without this the disk fills with artwork. Empty takes everything.
+* **`fetch_internal`** — internal addresses and networks that feeds, episodes, artwork and
+  `.torrent` files may still be fetched from, as `["192.168.1.20", "10.0.0.0/8"]`. Anything off
+  the public internet (loopback, the LAN, Docker's networks, link-local) is refused otherwise,
+  redirects included, because a feed's publisher or anyone signed in chooses what is fetched. List
+  addresses, not names: a feed on `nas.lan` needs `nas.lan`'s address here. The directory is not
+  affected. Empty by default; read at start.
 
 ## `[torrent]`