feat(meta,core,app,android,ios): AniSkip where files have no chapters

Most files that are not fansub releases carry no chapters, so they had no
skip button. AniSkip knows where an episode's opening and ending are, by
MyAnimeList id and episode number. ani.zip sends each entry's MyAnimeList
id and enrichment now keeps it on every episode row; schema v14 adds the
column and marks AniList titles un-enriched, deleting nothing, as v13 did.

When a file has loaded with no chapter naming an opening or ending, and
its length is known, each client asks once, with enrichment on only. The
answer lives as long as the file is open. with_skip_ranges cuts it into
the chapters as chapters named Opening and Ending, so the skip button,
auto-skip, the up-next countdown and the seek bar's marks all work
through the code a chaptered file uses. A file's own opening or ending
always wins over AniSkip's.

PRIVACY.md lists what is sent: the MyAnimeList id, the episode number and
the file's length.

docs/USER_GUIDE.md

@@ -135,7 +135,8 @@ An episode counts as watched once you have seen 90% of it.
 The picture plays inside the app's window. Move the mouse to show the controls.
 
 - **Skip opening** and **Skip ending** appear when the file's chapters mark an opening, an ending
-  or a preview. Files without chapter names get no skip button. Turn on **Skip openings and
+  or a preview. For an anime whose file marks neither, the app asks AniSkip where they are, with
+  **Posters and descriptions** on; they then show as chapters too. Turn on **Skip openings and
   credits** in Settings to skip them without asking.
 - **Up next** counts down 10 seconds when the credits start, then plays the next episode. Choose
   **Play now**, **Cancel** or **Watch till the end**.