Keep artwork on disk and serve every image from iPX (#111)

The page loaded artwork from each publisher's server, or through /api/art, fetched every time,
for http-only hosts. Nothing was kept, every visit asked every publisher, and artwork went when
a publisher's server did.

- The page draws every image a feed or item names from /api/art. The first time, iPX fetches
  it (only an address a feed or item names, only an image, up to 5 MB, within the feed timeout)
  and keeps it in art/ beside the database, under a hash of its address with its type beside
  it (src/art.rs). Later it comes from disk, which marks it as used.
- art_cache_mb, a server setting on the admin page, 500 by default, caps what is kept: the
  sweep before each scan drops the least recently shown until it fits. 0 keeps nothing, and
  artwork is fetched through iPX each time. Settings saved before it get the default.
- Served from iPX's own address, someone else's image must stay an image: nosniff, and a CSP
  with sandbox, so an SVG opened on its own runs no script as iPX.
- The fixture server sends .jpg as image/jpeg, which /api/art requires.

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

docs/configuration.md

@@ -4,7 +4,7 @@ Two places. **config.toml** holds what ipx needs before it reaches its database,
 who gets in: where things are (`download_dir`, `socket`, `organize`), `[torrent]` and `[web]`.
 **The database** holds the catalogue of feeds (`[feeds.<id>]` below) and the server settings the
 admin page edits (`schedule`, `max_total_gb`, `max_age_days`, `max_new_per_check`,
-`media_types`). Change those in the web UI, or with `ipx add`, `ipx rm` and `ipx import`; they
+`media_types`, `art_cache_mb`). Change those in the web UI, or with `ipx add`, `ipx rm` and `ipx import`; they
 take effect without a restart.
 
 The first time ipx meets a database that holds no catalogue, it takes the feeds and those
@@ -39,9 +39,10 @@ max_total_gb      = 50              # 0 = unlimited
 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
 ```
 
-`schedule`, `max_total_gb`, `max_age_days`, `max_new_per_check` and `media_types` move into the
+`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.
 
 * **`schedule`** — how often feeds are re-checked. A feed's own `<ttl>` still wins when it asks to
@@ -54,6 +55,10 @@ database as described above; `download_dir`, `socket` and `organize` stay in con
   read it. `0` disables it entirely.
 * **`max_age_days`** — items older than this with no file on disk are pruned from the database.
   Kept ones stay. `0` disables it.
+* **`art_cache_mb`** — how much show and episode artwork iPX keeps on disk, in `art/` beside the
+  database, so the page loads it from iPX rather than from every publisher's server. Over this,
+  what has gone longest unshown goes first. `0` keeps none: artwork still comes through iPX, fetched
+  each time. 500 by default.
 * **`max_new_per_check`** — how many of a feed's newest episodes are downloaded; older ones stay
   listed to download by hand. It stops a new subscription pulling a whole back catalogue.
   `0` means every episode, for an archive; set it on the feeds you want archived, since on the