Add ASGI support for the interactive debugger

Goal

Provide an ASGI counterpart to DebuggedApplication so ASGI applications (Starlette, FastAPI, and bare ASGI apps) can get the same in-browser interactive traceback debugger currently available only to WSGI applications.

Done criteria:

• A new AsgiDebuggedApplication middleware (e.g. backlash/asgi.py) that:
  • Passes through non-http scopes (websocket, lifespan) untouched.
  • Catches unhandled exceptions and renders the existing debugger page when the response has not started; falls back to log-only when http.response.start was already sent (mirroring the WSGI streamed-response fallback).
  • Serves the existing debugger endpoints (resource, source, cmd eval, /__console__) over ASGI, honoring scope["root_path"] for mounted apps.
• Reuse tbtools.py, console.py, and statics/ as-is; they are already transport-agnostic.
• Consider dropping the WebOb dependency in the same pass: debug.py is the only module importing it (query parsing plus small responses), and stdlib urllib.parse covers the need. This would make the package dependency-free and easier to vendor.

Why

The interactive debugger is the most valuable part of backlash, but it is WSGI-only. Modern Python web frameworks (FastAPI, Starlette, Shiny, Gradio) are ASGI, so they cannot use it today. Roughly 90% of the code (traceback capture/rendering, console eval, static assets) is transport-independent, so ASGI support is a thin adapter rather than a rewrite.

One known integration caveat worth documenting: Starlette's ServerErrorMiddleware is outermost in a Starlette app; it sends its own 500 response before re-raising, so an outer wrap sees the exception only after the response has started. Injecting via app.add_middleware(AsgiDebuggedApplication) places the debugger inside ServerErrorMiddleware but outside the routes, catching exceptions before any response starts.

References

• backlash/debug.py (DebuggedApplication) — WSGI dispatch logic to mirror; sole WebOb consumer.
• backlash/tbtools.py, backlash/console.py, backlash/statics/ — transport-agnostic pieces to reuse.
• Starlette ServerErrorMiddleware behavior: catches, responds 500, then re-raises — relevant to middleware placement.