feat(web): show which plugins use each font, warn before deleting one

The Fonts tab lists font files from the web process's own scan, and the
plugins that register fonts run in the display process, so the tab had no
way to say whether a font was in use before deleting it.

The display service now publishes {catalog key: [plugin ids]} to the
shared cache (font_usage_snapshot, src/font_usage.py), built from the
loaded plugins' FontManager.register_manager_font() registrations. A
daemon thread checks every 10 s and writes only when the usage changed
(plus a daily refresh so cache cleanup cannot expire it); it never raises.
Families, aliases (press_start, four_by_six, ...) and paths are resolved
through FontManager's catalog to the file stem the Fonts tab keys rows by;
fonts outside assets/fonts are left out. Unloading a plugin drops its
registrations (new FontManager.forget_manager_fonts).

GET /api/v3/fonts/catalog merges used_by into each row per request (the
5-minute scan cache is copied, never edited): a list of plugin ids, or
null when the display service has not reported. The tab shows a Used by
column ("unknown" / "-" / ids, rendered as text) and deleting an in-use
font names the plugins in the confirmation, from a fresh read. The server
still refuses only system fonts. Catalog fetches bypass the browser's
5-second API cache, which otherwise served the pre-delete list.

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

CHANGELOG.md

@@ -25,6 +25,12 @@ accepts both, but the store flags the old spelling as deprecated
   that will now render the font it asked for.
 - `src.wifi_manager.get_wifi_status_path()` — where WiFi status messages for
   the display are written (`config/wifi_status.json`).
+- The web UI's Fonts tab has a **Used by** column: the loaded plugins that
+  registered each font with `FontManager.register_manager_font()`, published
+  by the display service to the shared cache (`src/font_usage.py`) and merged
+  into `GET /api/v3/fonts/catalog` as `used_by`. Deleting a font a plugin
+  uses now names those plugins in the confirmation (it is not blocked).
+  `FontManager.forget_manager_fonts()` is new; unloading a plugin calls it.
 
 Deprecated, removed in 3.7.0 (each logs a warning on first use; see
 `docs/PLUGIN_API_REFERENCE.md#deprecated-apis` for replacements). Nothing in