Persist FFT settings, fix backend naming, parallelise batch DC caching

- FFT backend and pad factor persist across sessions via QSettings
  (IniFormat; tests redirect the settings path for hermeticity).
- The default backend was labelled "NumPy FFT" but always dispatched to
  scipy.fft — rename the canonical value to "scipy" ("numpy" stays as a
  legacy alias) and fix the dialog label.
- cache_file: DC caching fans out over angles via _parallel_map with
  per-angle budgets (the DcPrecomputeWorker pattern); FFT caching stays
  serial per angle because compute_rf_image now parallelises internally
  over blocks. Documented that the v7 FFT cache is natural-resolution
  (pad 1) by design.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/bench_fft.py

@@ -77,7 +77,7 @@ def main():
     p.add_argument("--rows", type=int, default=8)
     p.add_argument("--frames", type=int, default=1024)
     p.add_argument("--backends", default=None,
-                   help="comma-separated (default: numpy,pyfftw if available)")
+                   help="comma-separated (default: scipy,pyfftw if available)")
     p.add_argument("--real", help="path to a real .sras file")
     p.add_argument("--real-rows", type=int, default=32,
                    help="rows of angle 0 to use from the real file")
@@ -87,7 +87,7 @@ def main():
     if args.backends:
         backends = args.backends.split(",")
     else:
-        backends = ["numpy"] + (["pyfftw"] if compute.PYFFTW_AVAILABLE else [])
+        backends = ["scipy"] + (["pyfftw"] if compute.PYFFTW_AVAILABLE else [])
 
     if args.real:
         sras = row_slice(SrasFile(args.real), 0, args.real_rows)