feat(ttp): E.1.9 API contract — seven router endpoints, admin-gated state mutations, response models

Mounts /api/v1/ttp/* with empty-list / empty-Navigator responses.
GET endpoints viewer-gated; POST/DELETE /rules/{rule_id}/state
admin-gated server-side. POST parses JSON manually so a malformed
body returns the documented 400 (per feedback_schemathesis_400).

Drops xfail-strict markers from E.2.8 tests now that the router is
mounted; 26 tests pass against the contract handlers.

decnet/web/router/ttp/api_get_techniques.py

@@ -0,0 +1,34 @@
+"""GET /api/v1/ttp/techniques — distinct techniques observed fleet-wide.
+
+Returns an empty list at contract phase (E.1.9). Repo wiring lands in
+E.1.10 / E.3 implementation; the response shape is stable from here.
+"""
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import APIRouter, Depends
+
+from decnet.telemetry import traced as _traced
+from decnet.web.db.models import TechniqueRollupRow
+from decnet.web.dependencies import require_viewer
+
+router = APIRouter()
+
+
+@router.get(
+    "/ttp/techniques",
+    tags=["TTP Tagging"],
+    response_model=list[TechniqueRollupRow],
+    responses={
+        401: {"description": "Could not validate credentials"},
+        403: {"description": "Insufficient permissions"},
+    },
+)
+@_traced("api.ttp.list_techniques")
+async def api_list_techniques(
+    user: dict[str, Any] = Depends(require_viewer),
+) -> list[TechniqueRollupRow]:
+    """Distinct techniques observed across the fleet, with counts and
+    last-seen timestamps. Empty list at contract phase."""
+    return []