feat(airing-schedule): let admins override an anime's start season

[revam]

Blocked by #1497 (the Airing Schedule utility): this branch is built on it and shows its commits until it's merged; review only the last commit.

Why

The season view places each anime in the season its first regular episode aired in. That season comes from AniDB's dates, and sometimes it is wrong for a chart: a late-night premiere that falls a day before a season boundary, a show whose first episode went out at an event months early, or an anime with no dates yet. The core can now store a start season set by hand per AniDB anime. This PR lets admins set it from the season view and manage the overrides in the settings.

Where this sits

• Branch: feat/start-season-overrides, against master. It only adds to the season view and the airing schedule settings page that feat: add the Airing Schedule utility, read the dashboard's upcoming anime from it, and add its settings #1497 introduces, which is why it carries that PR's twelve commits for now.
• Commit to review: the last one, feat(airing-schedule): let admins override an anime's start season.
• Server: needs the server's start season override routes from the current core master:
  • GET Series/AniDB/{anidbID}/StartSeason: { Year, Season, IsOverridden, Computed: { Year, Season } | null }
  • PUT on the same path (admin) with { Year, Season } (year 1900 to 9999), and DELETE (admin), which clears the override
  • GET Series/AniDB/StartSeason/Overrides: [{ AnidbAnimeID, Title, Year, Season, CreatedAt, UpdatedAt, UserID }], where Title is the anime's preferred title, or null when the server has no data for it or the user may not see it
  • GET Series/AniDB/StartSeason/Overrides.csv (header AnidbAnimeID,Year,Season), and POST to it (admin) with a multipart file, answering { Added, Updated, Unchanged, Rejected: [{ Line, Text, Reason }] }
  • IsStartSeasonOverridden on each anime of GET AiringSchedule/Season/{year}/{season}/Sections

What

Season cards:

• For admins, a "Move to season…" icon button in the top-right corner of each card. It opens a small modal with a season picker and a year field. They start on the anime's current start season (from GET …/StartSeason), or on the viewed season when the anime has none.
• When the anime has an override, the modal says so and names the season its dates give ("Moved by hand. Its dates put it in Fall 2026.") and offers Reset to Computed (DELETE).
• Save (PUT) is enabled only for a different season with a valid year. Both actions toast, close the modal, and refetch the season sections, the season lists (the slider's and the season browser's counts) and the override list, so a moved anime leaves the view right away.
• One modal for the whole view (in SeasonView), not one per card.
• Cards whose start season was set by hand show a small calendar-lock marker with the tooltip "Moved to this season by hand", for every user.

Settings > Airing Schedule, a new admin-only Start Season Overrides section above Provider Priority:

• The number of overrides next to the heading.
• Export CSV downloads start_season_overrides.csv from the server's export.
• Import CSV is a file picker. The import's report shows under the buttons ("Imported: 1 updated, 1 unchanged, 2 rejected"), with each rejected line's number, its text and the server's reason.
• A compact list of the overrides: the AniDB ID (linked to AniDB), the title when the server has the anime, the season, and a remove button (DELETE).
• Like the channel merges, all of it saves at once instead of waiting for the page's Save, and the section's description says so.

Titles come with the override list itself, so the list makes no request per row. An anime the server has no data for shows "Unknown anime".

Look first at: src/components/Dialogs/StartSeasonModal.tsx, src/components/Settings/AiringScheduleSettings/StartSeasonOverrides.tsx, src/components/Utilities/AiringSchedule/SeasonCard.tsx, and the new queries and mutations at the end of src/core/react-query/airing-schedule/queries.ts and mutations.ts. Types are in src/core/types/api/airing-season.ts, pure helpers in src/core/utilities/startSeason.ts.

Feedback wanted

• The card shows two icons when an admin looks at an overridden anime (the marker and the move button). Should the marker be folded into the button for admins, for example by colouring the button?
• Should the modal warn when the picked season is the computed one, and suggest Reset instead of saving an override equal to it?

Testing

• On the tip: pnpm lint, pnpm tscheck, pnpm test (262 passed; new: tests/core/utilities/startSeason.test.ts for the year range, the picker's starting season and the import summary line) and pnpm build all pass. No dependency changes.
• Checked live against a local server on core master, as an admin, before the branch moved onto master:
  • Move: in Fall 2026, "Move to season…" on Tensei Shitara Ken Deshita II (AniDB 17789) opened on Fall 2026. Picking Winter 2027 and saving stored the override (GET …/Overrides), and Fall 2026 refetched from 34 to 33 anime. In Winter 2027 the card showed the marker, and its modal showed "Moved by hand. Its dates put it in Fall 2026." with Reset to Computed.
  • Reset: Reset to Computed in the modal cleared the override and the card left Winter 2027.
  • Import: importing a CSV with a new line, an unchanged line, a bad ID and a year of 1850 reported the counts and both rejected lines with the server's reasons. A second file with a changed season showed "1 updated".
  • Export: downloaded start_season_overrides.csv with the header and the two rows.
  • Remove: the remove buttons emptied the list, and the count went to 0.
• Not yet live: the titles from the override list. The live checks above ran while the list still fetched a title per row; the switch to Title ("Unknown anime" for null) was checked by reading the code.
• Not tried: a user who is not an admin. The button and the section are gated on useCurrentUserQuery().data?.IsAdmin like the channel merges; the marker shows for everyone.