feat(backend): BackendStatus enum, quiet status(), is_backend_ready()

Add a module-level BackendStatus(IntEnum) with READY=0 to replace magic
0 comparisons. Extend BottleBackend.status() with a quiet parameter:
quiet=False (default) prints diagnostics; quiet=True returns the code
silently for programmatic checks.

Add is_backend_available() (cheap PATH check, alias for has_backend) and
is_backend_ready(name, *, quiet=False) (full status() check) to the
backend package for callers that need to distinguish availability from
readiness.

Update tests/_backend.py guards to use is_backend_ready(quiet=False) so
diagnostic output is printed during test discovery, giving the operator a
concrete reason for each skip rather than a bare "prerequisites
unavailable" message.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/backend/docker/backend.py

@@ -20,7 +20,8 @@ infrastructure: CA install and git copy-in.
 from __future__ import annotations
 
 import shutil
-from contextlib import contextmanager
+import io
+from contextlib import contextmanager, redirect_stderr
 from pathlib import Path
 from typing import Generator, Sequence
 
@@ -60,8 +61,11 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
         return _setup.setup()
 
     @classmethod
-    def status(cls) -> int:
+    def status(cls, *, quiet: bool = False) -> int:
         from . import setup as _setup
+        if quiet:
+            with redirect_stderr(io.StringIO()):
+                return _setup.status()
         return _setup.status()
 
     @classmethod