feat(deckies): generic file drops on fleet + MazeNET deckies

Extracts the docker-exec-with-base64-stdin pattern out of canary/planter
and orchestrator/drivers/ssh into a shared decnet.decky_io package.
Both consumers now delegate; the canary planter test still proves the
contract end-to-end.

Adds POST/DELETE /api/v1/deckies/files for arbitrary file drops.
Container resolution is shared with the canary path: topology_id absent
means fleet (<name>-ssh), present routes through resolve_decky_container
which picks <name>-ssh when the topology decky exposes ssh, else the
topology base container decnet_t_<id8>_<name>.

Path validation rejects relative paths and '..' traversal at the request
model layer.  Bad base64 → 400; unknown topology → 404; decky not in
topology → 422; docker exec failure → 409.

decnet/web/db/models/__init__.py

@@ -63,6 +63,10 @@ from .deploy import (
     MutateIntervalRequest,
     PurgeResponse,
 )
+from .decky import (
+    DeckyFileDeleteRequest,
+    DeckyFileDropRequest,
+)
 from .fleet import (
     LOCAL_HOST_SENTINEL,
     FleetDecky,
@@ -222,6 +226,8 @@ __all__ = [
     "PurgeResponse",
     # fleet
     "LOCAL_HOST_SENTINEL",
+    "DeckyFileDeleteRequest",
+    "DeckyFileDropRequest",
     "FleetDecky",
     # health
     "ComponentHealth",

decnet/web/db/models/decky.py

@@ -0,0 +1,61 @@
+"""DTOs for cross-cutting decky operations (file drops, etc.).
+
+These don't bind to a single table — fleet deckies and MazeNET
+(topology) deckies share the request shape, with ``topology_id``
+discriminating.  Following ``feedback_models_single_source`` we put
+the request/response shapes alongside the rest of the API contracts
+under ``decnet.web.db.models``.
+"""
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel, Field as PydanticField, field_validator
+
+
+class DeckyFileDropRequest(BaseModel):
+    """Drop arbitrary bytes at an absolute path inside a decky container.
+
+    ``content_b64`` is the base64-encoded payload.  Binary-safe.
+
+    ``mode`` defaults to ``0o644`` (octal int).  ``mtime_offset`` is a
+    seconds offset from now applied via ``touch -d`` so realistic-aged
+    files don't all stamp at wall-clock-now.
+    """
+    decky_name: str = PydanticField(..., min_length=1)
+    topology_id: Optional[str] = None
+    path: str = PydanticField(..., min_length=1)
+    content_b64: str
+    mode: int = 0o644
+    mtime_offset: int = 0
+
+    @field_validator("path")
+    @classmethod
+    def _abs_no_traversal(cls, v: str) -> str:
+        if not v.startswith("/"):
+            raise ValueError("path must be absolute (start with '/')")
+        # Defense in depth: even though we run as root inside the
+        # container, ``..`` segments make the on-disk location depend
+        # on the cwd at exec-time and surprise both operators and the
+        # auditor reading the placement_path field later.
+        for seg in v.split("/"):
+            if seg == "..":
+                raise ValueError("path must not contain '..' segments")
+        return v
+
+
+class DeckyFileDeleteRequest(BaseModel):
+    """Best-effort ``rm -f`` of an absolute path inside a decky container."""
+    decky_name: str = PydanticField(..., min_length=1)
+    topology_id: Optional[str] = None
+    path: str = PydanticField(..., min_length=1)
+
+    @field_validator("path")
+    @classmethod
+    def _abs_no_traversal(cls, v: str) -> str:
+        if not v.startswith("/"):
+            raise ValueError("path must be absolute (start with '/')")
+        for seg in v.split("/"):
+            if seg == "..":
+                raise ValueError("path must not contain '..' segments")
+        return v