refactor(models): split models.py into topical submodules

decnet/web/db/models.py was approaching 1000 lines across User/Log/
Attacker/Swarm/Topology/Workers/Updater/Health domains. Split into a
package with one module per domain; __init__.py re-exports every symbol
so all 52 call sites keep importing from decnet.web.db.models
unchanged.

decnet/web/db/models/workers.py

@@ -0,0 +1,50 @@
+"""Workers panel DTOs (bus-backed health + control)."""
+from typing import Any, Dict, List, Literal, Optional
+
+from pydantic import BaseModel, Field as PydanticField
+
+
+# --- Workers panel (Config → Workers) ---
+# Bus-backed health + control: workers heartbeat on ``system.<name>.health``
+# and listen on ``system.<name>.control``.  The API aggregates last-seen
+# heartbeats via the worker registry; these are the HTTP-facing shapes.
+
+class WorkerStatus(BaseModel):
+    name: str
+    # ``ok`` — heartbeat within 90s (3× 30s heartbeat interval)
+    # ``stale`` — worker was seen before but hasn't pulsed in 90s+
+    # ``unknown`` — we've never received a heartbeat from this name
+    status: Literal["ok", "stale", "unknown"]
+    last_heartbeat_ts: Optional[float] = None
+    seconds_since: Optional[float] = None
+    # Whatever the worker's ``extra()`` callback put in the heartbeat;
+    # opaque to the panel, displayed only if the UI knows the key.
+    extra: Dict[str, Any] = PydanticField(default_factory=dict)
+    # True iff a ``decnet-<name>.service`` unit file is present on the
+    # host.  False flips the UI START button to disabled with a
+    # "Unit not installed" tooltip.  Default True for backwards compat
+    # on clients that pre-date the field.
+    installed: bool = True
+
+
+class WorkersResponse(BaseModel):
+    workers: List[WorkerStatus]
+    generated_at: float
+    bus_connected: bool
+
+
+class WorkerControlResponse(BaseModel):
+    accepted: bool
+    worker: str
+    action: str
+
+
+class StartFailure(BaseModel):
+    name: str
+    reason: str
+
+
+class StartAllResponse(BaseModel):
+    started: List[str]
+    already_running: List[str]
+    failed: List[StartFailure]