feat(swarm): remote teardown API + UI (per-decky and per-host)

Agents already exposed POST /teardown; the master was missing the plumbing
to reach it. Add:

- POST /api/v1/swarm/hosts/{uuid}/teardown — admin-gated. Body
  {decky_id: str|null}: null tears the whole host, a value tears one decky.
  On worker failure the master returns 502 and leaves DB shards intact so
  master and agent stay aligned.
- BaseRepository.delete_decky_shard(name) + sqlmodel impl for per-decky
  cleanup after a single-decky teardown.
- SwarmHosts page: "Teardown all" button (keeps host enrolled).
- SwarmDeckies page: per-row "Teardown" button.

Also exclude setuptools' build/ staging dir from the enrollment tarball —
`pip install -e` on the master generates build/lib/decnet_web/node_modules
and the bundle walker was leaking it to agents. Align pyproject's bandit
exclude with the git-hook invocation so both skip decnet/templates/.

decnet/web/router/swarm_mgmt/__init__.py

@@ -15,6 +15,7 @@ from .api_list_hosts import router as list_hosts_router
 from .api_decommission_host import router as decommission_host_router
 from .api_list_deckies import router as list_deckies_router
 from .api_enroll_bundle import router as enroll_bundle_router
+from .api_teardown_host import router as teardown_host_router
 
 swarm_mgmt_router = APIRouter(prefix="/swarm")
 
@@ -22,3 +23,4 @@ swarm_mgmt_router.include_router(list_hosts_router)
 swarm_mgmt_router.include_router(decommission_host_router)
 swarm_mgmt_router.include_router(list_deckies_router)
 swarm_mgmt_router.include_router(enroll_bundle_router)
+swarm_mgmt_router.include_router(teardown_host_router)

decnet/web/router/swarm_mgmt/api_enroll_bundle.py

@@ -55,6 +55,9 @@ _EXCLUDES: tuple[str, ...] = (
     ".pytest_cache", ".pytest_cache/*",
     ".mypy_cache", ".mypy_cache/*",
     "*.egg-info", "*.egg-info/*",
+    # setuptools build/ staging dir — created by `pip install` and leaks a
+    # nested decnet_web/node_modules/ copy into the bundle otherwise.
+    "build", "build/*", "build/**",
     "*.pyc", "*.pyo",
     "*.db", "*.db-wal", "*.db-shm", "decnet.db*",
     "*.log",

decnet/web/router/swarm_mgmt/api_teardown_host.py

@@ -0,0 +1,70 @@
+"""POST /swarm/hosts/{uuid}/teardown — remote teardown on a swarm worker.
+
+Body: ``{"decky_id": "..."}`` (optional). When ``decky_id`` is null/omitted
+the agent tears down the entire host (all deckies + network); otherwise it
+tears down that single decky. Mirrors the arguments of the local
+``decnet teardown`` CLI command.
+"""
+from __future__ import annotations
+
+from typing import Optional
+
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
+
+from decnet.logging import get_logger
+from decnet.swarm.client import AgentClient
+from decnet.web.db.repository import BaseRepository
+from decnet.web.dependencies import get_repo, require_admin
+
+log = get_logger("swarm.teardown")
+router = APIRouter()
+
+
+class TeardownHostRequest(BaseModel):
+    decky_id: Optional[str] = None
+
+
+class TeardownHostResponse(BaseModel):
+    host_uuid: str
+    host_name: str
+    decky_id: Optional[str] = None
+    ok: bool
+    detail: str
+
+
+@router.post(
+    "/hosts/{uuid}/teardown",
+    response_model=TeardownHostResponse,
+    tags=["Swarm Management"],
+)
+async def teardown_host(
+    uuid: str,
+    req: TeardownHostRequest,
+    admin: dict = Depends(require_admin),
+    repo: BaseRepository = Depends(get_repo),
+) -> TeardownHostResponse:
+    host = await repo.get_swarm_host_by_uuid(uuid)
+    if host is None:
+        raise HTTPException(status_code=404, detail="host not found")
+
+    try:
+        async with AgentClient(host=host) as agent:
+            body = await agent.teardown(req.decky_id)
+    except Exception as exc:
+        log.exception("swarm.teardown dispatch failed host=%s decky=%s",
+                      host.get("name"), req.decky_id)
+        raise HTTPException(status_code=502, detail=str(exc)) from exc
+
+    if req.decky_id:
+        await repo.delete_decky_shard(req.decky_id)
+    else:
+        await repo.delete_decky_shards_for_host(uuid)
+
+    return TeardownHostResponse(
+        host_uuid=uuid,
+        host_name=host.get("name") or "",
+        decky_id=req.decky_id,
+        ok=True,
+        detail=str(body),
+    )