refactor: wire web to the in-repo engine and shared data dir

- web/src/server/paths.ts resolves the repo root from its own location,
  loads the single repo-root .env, and defines data/web/ (watchlist,
  stock cache, backtest cache) and data/precomputed/.
- The API spawns bridge.py at the repo root instead of ../quant.
- precompute_backtest.py writes to data/precomputed/ instead of
  ../stock-agent/data/precomputed.
- Rename the project to quant-trading (web package quant-trading-web).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

bridge.py

@@ -1,5 +1,5 @@
 """
-Bridge script for stock-agent integration.
+Bridge between the Python engine and the web API (web/src/server/strategyService.ts).
 
 Outputs JSON to stdout; all logs go to stderr.
 
@@ -1186,7 +1186,7 @@ def cmd_live(args: argparse.Namespace) -> None:
 # ---------------------------------------------------------------------------
 
 def main() -> None:
-    parser = argparse.ArgumentParser(description="Quant bridge for stock-agent")
+    parser = argparse.ArgumentParser(description="Quant bridge for the web API")
     subparsers = parser.add_subparsers(dest="command")
 
     # list