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/health.py

@@ -0,0 +1,14 @@
+"""Health-endpoint DTOs."""
+from typing import Literal, Optional
+
+from pydantic import BaseModel
+
+
+class ComponentHealth(BaseModel):
+    status: Literal["ok", "failing"]
+    detail: Optional[str] = None
+
+
+class HealthResponse(BaseModel):
+    status: Literal["healthy", "degraded", "unhealthy"]
+    components: dict[str, ComponentHealth]