chore(deprecation): retarget the 35 deprecations to 3.8.0 and add a usage scan (#681)

Moves the 35 @deprecated markers from 3.7.0 (already shipped with them in
place) to 3.8.0, and adds scripts/plugin_api_usage.py plus the generated
docs/DEPRECATIONS_3.8.md: who still calls or overrides each deprecated
method across core, the monorepo and third-party plugins. Removes nothing.

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

docs/DEVELOPER_QUICK_REFERENCE.md

@@ -54,7 +54,7 @@ rows = self.layout.bounds.inset(1).split_v(3, 1, gap=1)
 self.draw_fit("12:34", rows[0])                 # largest crisp font that fits
 self.draw_image(logo, rows[1], mode="fill_height", crop_to_ink=True)
 
-# Weather icons: draw_weather_icon() is deprecated, removed in 3.7.0 —
+# Weather icons: draw_weather_icon() is deprecated, removed in 3.8.0 —
 # draw your own icons (the weather plugin ships WeatherIcons)
 
 # Scrolling state
@@ -78,7 +78,7 @@ strategy = cache_manager.get_cache_strategy("weather")
 ```
 
 `get_background_cached_data()` (use `get()`) and `get_sport_live_interval()`
-are deprecated, removed in 3.7.0. See
+are deprecated, removed in 3.8.0. See
 [Deprecated APIs](PLUGIN_API_REFERENCE.md#deprecated-apis).
 
 ## Plugin Manager Quick Methods
@@ -87,7 +87,7 @@ are deprecated, removed in 3.7.0. See
 # Get plugins
 plugin = plugin_manager.get_plugin("plugin-id")
 all_plugins = plugin_manager.get_all_plugins()
-# get_enabled_plugins() is deprecated, removed in 3.7.0 — check `enabled`
+# get_enabled_plugins() is deprecated, removed in 3.8.0 — check `enabled`
 # on the entries in plugin_manager.plugins
 
 # Get info