Converter: cover real Ren'Py games better, add folder import and dry run

Checked against six real games (Ren'Py 7.5 and 8.x):
- nested labels and named menus become jump anchors inside label bodies
- call with arguments and labels with parameters (dynamically scoped)
- frame-by-frame ATL images, Movie frames, layeredimage, camera, Shake,
  custom audio channels via register_channel, show text with expressions
- generator expressions in Python, no SyntaxWarning noise from decompiled code

Also:
- app.import_dir converts an unpacked game folder without changing it
- app.converter.analyze dry-runs the script part of the converter
- imports use their own "importing" status so a worker restart can't hijack them

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/converter/rpa.py

@@ -7,6 +7,7 @@ unpickler that only allows plain containers — archives are untrusted input.
 import io
 import pickle
 import zlib
+from collections.abc import Callable
 from pathlib import Path
 
 
@@ -65,13 +66,15 @@ def read_index(path: Path) -> dict[str, list[tuple[int, int, bytes]]]:
     return index
 
 
-def extract_rpa(path: Path, dest: Path) -> int:
-    """Extract all files of an archive into dest. Returns the number of files."""
+def extract_rpa(path: Path, dest: Path, only: Callable[[str], bool] | None = None) -> int:
+    """Extract files of an archive into dest (all, or those `only` accepts)."""
     index = read_index(path)
     root = dest.resolve()
     count = 0
     with path.open("rb") as f:
         for name, entries in index.items():
+            if only is not None and not only(name):
+                continue
             target = (dest / name).resolve()
             if not target.is_relative_to(root):
                 continue