feat(api): pin response_model on dict-returning mutation routes

Every mutation route that returned an untyped dict now declares
response_model at the decorator. MessageResponse covers the eight
{"message": ...} envelopes (change-password, mutate-decky, mutate-
interval, update-deployment-limit, update-global-mutation-interval,
delete-user, update-user-role, reset-user-password). Purpose-built
models cover the richer shapes (DeployResponse for /deckies/deploy,
PurgeResponse for /config/reinit, ReapReportResponse for /reap-orphans,
UserResponse for /config/users). 204-No-Content and Response/
ORJSONResponse routes stay as-is.

The wire shape for clients is unchanged — the envelopes already only
shipped a message field. What changes is that a handler which
accidentally returns a richer dict (e.g. a full user row including
password_hash) would be silently stripped to the declared fields at
serialization time.

Also flips F4/D "expensive LIKE" to accepted (new DA-09) — the /logs
and /attackers search routes LIKE-scan unbounded columns, but both are
admin-gated, limit-capped, and operator rate-limit scope per DA-04.
FTS5 stays a performance TODO, not a security blocker.

decnet/web/router/fleet/api_mutate_interval.py

@@ -3,7 +3,7 @@ from fastapi import APIRouter, Depends, HTTPException
 from decnet.telemetry import traced as _traced
 from decnet.config import DecnetConfig
 from decnet.web.dependencies import require_admin, repo
-from decnet.web.db.models import MutateIntervalRequest
+from decnet.web.db.models import MessageResponse, MutateIntervalRequest
 
 router = APIRouter()
 
@@ -17,6 +17,7 @@ def _parse_duration(s: str) -> int:
 
 
 @router.put("/deckies/{decky_name}/mutate-interval", tags=["Fleet Management"],
+    response_model=MessageResponse,
     responses={
         400: {"description": "Bad Request (e.g. malformed JSON)"},
         401: {"description": "Could not validate credentials"},