refactor: remove unused SportMonks integration from the public repo

It was never wired into scoring (weight-0 player-SoT sidecar + a never-called
opponent-adjustment experiment), so this changes ZERO predictions — proven: the
only pricing-path edit removes _apply_sm_sot, which was hard-gated off
(_USE_SM_PLAYER_SOT=False / _SM_SOT_WEIGHT=0.0 -> always returned immediately);
186 tests pass; validators 104/0.

- Removed the unused client + fetch scripts + opponent-adjustment experiment
  (bot/data/sportmonks.py, match_corpus.py, scripts/fetch_*, relevance_backtest.py,
  research/PAID_API.md) -- recoverable from git history.
- Un-wired the weight-0 SoT sidecar in teamstats.py.
- Produced data kept locally under neutral names (data/api_cache/,
  data/players/player_sot_rates.json), gitignored -- nothing lost.
- Scrubbed the brand name -> 'the stats API' across the remaining docs/data/config.
- git grep -i sportmonks in tracked files -> 0.

Author: DataAthleteChamp

.gitignore

@@ -19,7 +19,8 @@ data/history/.players_acc.json
 data/history/.players_done.json
 .venv/
 data/footballdata/
-data/sportmonks/
+data/api_cache/
+data/players/player_sot_rates.json
 
 # Dry-run prediction run-dumps (ISO-timestamped); keep named files + baselines/
 predictions/*T*Z.jsonl

bot/data/match_corpus.py

@@ -120,7 +120,7 @@ def team_strengths(matches: list[Match], stat: str = "goals") -> dict[str, float
 
     A simple, leakage-stable team attribute used only as the *opponent-similarity variable* (which
     past opponents resemble the current one). ``stat`` selects the differential: "goals" (history,
-    where goal-difference is available) or "shots_on_target" (the sportmonks corpus has no goals, so
+    where goal-difference is available) or "shots_on_target" (the stats-api corpus has no goals, so
     SoT-differential is the available dominance proxy). Z-scoring puts sigma on a ~unit scale.
     """
     diff: dict[str, list[float]] = defaultdict(list)