feat: fix mods tab, add client/server split, and scaffold server dirs

Mods tab bug fixes:
- mod_manager: fix wrong kwargs in set_enabled_mods, fix scan dir to use
  mods/ subdir instead of server root, migrate old string-list format to
  dict format on read
- service: replace dead server_mods SQL JOIN with get_enabled_mods()
  call through the mod_manager capability; pass is_server_mod to
  build_mod_args
- mods_router: accept list[EnabledModEntry] objects (name + is_server_mod)
  instead of bare strings

Client/server mod split:
- Mods now stored as list[{"name": str, "is_server_mod": bool}]; old
  string-list format auto-migrated on read
- is_server_mod=true routes to -serverMod= arg; false to -mod= arg
- ModList UI: amber Client/Server badge in selected pane; toggle button
  in split-pane selector

Directory scaffold:
- process_config: adds "mods" to dir layout; provides get_dir_readme()
  with per-directory README.txt content
- file_utils: ensure_server_dirs() gains readme_provider kwarg; writes
  README.txt idempotently if absent
- service.create_server: passes readme_provider via hasattr probe
- main.py startup: backfills all existing servers with correct subdirs
  and README files (idempotent)

Docs: API.md and FRONTEND.md updated for new mod schema and types
Test __init__.py files added for pytest discovery

backend/main.py

@@ -90,7 +90,24 @@ async def lifespan(app: FastAPI):
             except Exception as exc:
                 logger.error("Failed to reattach threads for server %d: %s", server["id"], exc)
 
-    # 8. Seed default admin if no users exist
+    # 8. Backfill server directory scaffold for existing servers (idempotent)
+    from core.dal.server_repository import ServerRepository as _ServerRepo
+    from core.utils.file_utils import ensure_server_dirs as _ensure_dirs
+    from adapters.registry import GameAdapterRegistry as _Registry
+    with engine.connect() as db:
+        for server in _ServerRepo(db).get_all():
+            try:
+                _adapter = _Registry.get(server["game_type"])
+                _pc = _adapter.get_process_config()
+                _ensure_dirs(
+                    server["id"],
+                    _pc.get_server_dir_layout(),
+                    readme_provider=getattr(_pc, "get_dir_readme", None),
+                )
+            except Exception as exc:
+                logger.warning("Dir scaffold failed for server %d: %s", server["id"], exc)
+
+    # 9. Seed default admin if no users exist
     from core.auth.service import AuthService
     with engine.connect() as db:
         svc = AuthService(db)
@@ -104,7 +121,7 @@ async def lifespan(app: FastAPI):
             logger.warning("  Change this password immediately!")
             logger.warning("=" * 60)
 
-    # 9. Register and start APScheduler cleanup jobs
+    # 10. Register and start APScheduler cleanup jobs
     from core.jobs.scheduler import start_scheduler, stop_scheduler
     from core.jobs.cleanup_jobs import register_cleanup_jobs
     register_cleanup_jobs()