feat: persist backend in BottleMetadata; use it in resume and dashboard reattach (PRD 0040)

BottleMetadata gains a backend field (default ""). Docker prepare writes
"docker"; smolmachines prepare writes "smolmachines". read_metadata
deserialises it with "" as the backward-compatible default.

resume now passes metadata.backend to _launch_bottle so a preserved
smolmachines bottle is resumed on the right backend without requiring
BOT_BOTTLE_BACKEND to be set manually.

_bottle_for_slug now reads metadata.backend and constructs a
SmolmachinesBottle for smolmachines slugs instead of always defaulting
to DockerBottle. No-metadata slugs still fall back to Docker.

Closes #137

bot_bottle/backend/smolmachines/prepare.py

@@ -71,9 +71,8 @@ def resolve_plan(
         cwd=spec.user_cwd if spec.copy_cwd else "",
         copy_cwd=spec.copy_cwd,
         started_at=datetime.now(timezone.utc).isoformat(),
-        # No compose project for smolmachines bottles; chunk 4
-        # will give dashboard discovery a backend-specific path.
         compose_project="",
+        backend="smolmachines",
     ))
 
     subnet, gateway, bundle_ip = smolmachines_bundle_subnet(slug)