parallelize the backtest across worker processes (true multi-core)

The replay was CPU-bound and single-core: the earlier asyncio.to_thread offload
kept the API responsive but, because of the GIL, ran on one core. Per-ticker
replay is independent, so fan it out across worker processes (which sidestep the
GIL) for real multi-core speedup.

- New `settings.backtest_workers` (default 4), capped to cpu_count-1 so a core
  stays free for the web server.
- Uses a `forkserver` context (workers forked from a clean single-threaded
  server — avoids the fork-with-threads deadlock); falls back to `fork`. On
  spawn-only platforms (Windows) and for 1-ticker runs it uses the thread path,
  so dev/tests are unaffected.
- Worker takes primitive column arrays (cheap to pickle), rebuilds bars, and
  returns (candidates, plain-dict signal series) — both picklable across the
  process boundary. Bars are still fetched in the event loop (ORM-safe).
- Pool creation is guarded: if the pool can't start, the job falls back to the
  sequential thread path instead of failing.

334 backend tests pass (parallel path is POSIX/server-only, so it's covered by
construction + the picklability/worker-count tests; the thread fallback is
exercised by the run_backtest smoke test).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/config.py

@@ -73,6 +73,12 @@ class Settings(BaseSettings):
     # backtests become computable. ~252 trading days/year.
     ohlcv_history_days: int = 1825
 
+    # Backtest parallelism: replay tickers across this many worker processes on
+    # POSIX (forkserver), capped to cpu_count-1 so a core stays free for the web
+    # server. 1 disables it (sequential). No effect on Windows / spawn-only
+    # platforms — those fall back to a single worker thread.
+    backtest_workers: int = 4
+
     # Database Pool
     db_pool_size: int = 5
     db_pool_timeout: int = 30