feat(ttp): E.3.4 API handlers wired to repo (rollups + Navigator)

Five GET rollup endpoints (techniques, by-identity, by-attacker,
by-campaign, by-session) and the Navigator export (fleet +
per-identity) now call into the TTPMixin methods. Rule catalogue
endpoint still returns [] — backed by the RuleStore which lands
at E.3.5/E.3.6.

decnet/web/router/ttp/api_get_by_session.py

@@ -7,7 +7,7 @@ from fastapi import APIRouter, Depends
 
 from decnet.telemetry import traced as _traced
 from decnet.web.db.models import IdentityTechniqueRow
-from decnet.web.dependencies import require_viewer
+from decnet.web.dependencies import repo, require_viewer
 
 router = APIRouter()
 
@@ -27,5 +27,5 @@ async def api_ttp_by_session(
     session_id: str,
     user: dict[str, Any] = Depends(require_viewer),
 ) -> list[IdentityTechniqueRow]:
-    """Per-session TTP tag timeline. Empty at contract phase."""
-    return []
+    """Per-session TTP tag timeline."""
+    return await repo.list_techniques_by_session(session_id)