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>

precompute_backtest.py

@@ -326,7 +326,7 @@ def main():
     parser = argparse.ArgumentParser(description="Pre-compute backtest results")
     parser.add_argument(
         "--output", default=None,
-        help="Output directory (default: ../stock-agent/data/precomputed)",
+        help="Output directory (default: data/precomputed, read by the web UI)",
     )
     parser.add_argument(
         "--years", type=int, default=10,
@@ -335,7 +335,7 @@ def main():
     args = parser.parse_args()
 
     output_dir = args.output or os.path.join(
-        os.path.dirname(os.path.abspath(__file__)), "..", "stock-agent", "data", "precomputed"
+        os.path.dirname(os.path.abspath(__file__)), "data", "precomputed"
     )
     os.makedirs(output_dir, exist_ok=True)