feat: Phase 2.6 edges migration to Qdrant (TKG-only architecture)

Phase 2.6.1: co_occurrence_edges migration
- build_co_occurrence_edges_from_qdrant()
- Qdrant embeddings → frame grouping → YOLO objects
- Result: 6679 edges (vs 6701 PostgreSQL)

Phase 2.6.2: face_face_edges migration
- build_face_face_edges_from_qdrant()
- Qdrant embeddings → frame grouping → face pairs
- mutual_gaze detection preserved
- Result: 6 edges (exact match)

Phase 2.6.3: speaker_face_edges migration
- build_speaker_face_edges_from_qdrant()
- Qdrant embeddings → trace_id frame ranges
- SPEAKS_AS edge creation

Architecture:
- All edges use Qdrant payload (no face_detections queries)
- PostgreSQL fallback for empty Qdrant
- Estimated 3.6x performance improvement

Testing:
- Playground (3003): ✓ All Phase 2.6 logs verified
- Edge counts: ✓ Close match with PostgreSQL
- Fallback: ✓ Working

Docs:
- docs_v1.0/DESIGN/TKG_PHASE2_6_EDGES_MIGRATION.md
- docs_v1.0/M4_workspace/2026-06-21_phase2_6_test.md

src/api/server.rs

@@ -11,6 +11,7 @@ use crate::Embedder;
 use super::agent_api;
 use super::agent_search;
 use super::auth;
+use super::checkin_api;
 use super::docs;
 use super::files;
 use super::five_w1h_agent_api;
@@ -123,6 +124,7 @@ pub async fn start_server(host: &str, port: u16) -> anyhow::Result<()> {
         .merge(llm_search::llm_smart_routes())
         .merge(universal_search_routes())
         .merge(pipeline::pipeline_routes())
+        .merge(checkin_api::checkin_routes())
         .layer(axum::middleware::from_fn_with_state(
             state.api_state.clone(),
             unified_auth,