Add ATP/WTA tennis provider (Live Tennis API) — issue #230

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Implements the tennis provider proposed in #230, targeting development as requested.

Disclosure: this provider is contributed by the Live Tennis API team (we run https://livetennisapi.com) — vendor-authored, so please judge it on the merits. The README section says the same up front.

What this adds

• providers/LiveTennisAPI.js — a new provider following the existing CPL.js / PWHL.js conventions: a singleton shared-cache poller (POLL_FREQUENCY, startDataPoll, getData) that normalizes each match into the module's existing game object (hTeam/vTeam/hTeamLong/vTeamLong/hScore/vScore/status/gameMode) and returns via the same getScores(payload, gameDate, callback) contract as PWHL (callback(games, sortIdx, noGamesToday)).
• node_helper.js — registers the provider (one line, alongside CPL/PWHL).
• MMM-MyScoreboard.js — adds ATP and WTA to the league map. Nothing else changes; no other sport or provider is modified.
• README.md — adds ATP/WTA to the leagues list and a "Tennis (ATP / WTA)" section covering config, the API key, and the rate limits.
• package.json — adds Tennis/ATP/WTA keywords.
• Unit tests — tests/unit/providers/LiveTennisAPI.test.js (12 tests).

Design (per the two questions in #230)

1. Config shape — tennis is player-based, so the existing teams array holds player surnames (teams: ["Alcaraz", "Gauff"]). No schema change. Omit teams to show the whole tour slate.
2. Rendering — the zero-renderer-change option: the home/visitor score slots show sets won; the set-by-set games (e.g. 6-4 3-6 2-1), current points and the serving player go in the existing status line. Players map to the home/visitor slots and the leagues set homeTeamFirst, so it renders "Player 1 vs Player 2".

API key & rate limits (documented in the README)

• The key is read server-side only from the LIVETENNIS_API_KEY environment variable and is never sent to the browser.
• Free tier is 30 requests/minute and 100 requests/day. The provider keeps one shared cache, so call volume does not grow with the number of tennis leagues or the module refresh rate. Each refresh spends 3 requests per tour (live + upcoming + completed); the default 15-minute cadence is ~288/day for one tour, which exceeds the free 100/day cap — so a free key is best for testing/light use and continuous operation needs a paid tier. LIVETENNIS_POLL_INTERVAL (minutes) lets a free key stay inside the cap (e.g. 45 min ≈ 96/day for one tour). A free key: https://livetennisapi.com/subscribe/free
• If the key is missing or throttled, tennis just shows no games and the rest of the scoreboard is unaffected.

Checks

• eslint clean across the whole repo (matches the module's @stylistic config).
• Unit tests: 125 pass / 0 fail (12 new). node --check passes on all touched JS.
• I could not run test:e2e here (it downloads a MagicMirror bootstrap + Chromium); happy to iterate if CI surfaces anything.

I don't follow racquet sports either, so I've kept this self-contained and additive — glad to adjust naming (e.g. a combined TENNIS league) or rendering to your preference.