CUB-123: integrate gateway, wire PostgreSQL repositories, add SSE streaming

- Create repository/ package with pgx-backed CRUD for agents, sessions, tasks, projects
- Define AgentRepo/SessionRepo/TaskRepo/ProjectRepo interfaces
- Update handler to use repository interfaces instead of in-memory stores
- Add SSE broker with GET /api/events endpoint (text/event-stream)
- Add gateway client that polls OpenClaw for agent states
- Add GATEWAY_URL and GATEWAY_POLL_INTERVAL config fields
- Seed 5 demo agents (Otto, Rex, Dex, Hex, Pip) on empty DB
- Update router to wire SSE broker
- All 21 handler tests pass with mock repos

go-backend/internal/handler/session.go

@@ -1,6 +1,7 @@
 package handler
 
 import (
+	"log/slog"
 	"net/http"
 
 	"code.cubecraftcreations.com/CubeCraft-Creations/Control-Center/go-backend/internal/models"
@@ -10,7 +11,12 @@ import (
 
 // ListSessions handles GET /api/sessions.
 func (h *Handler) ListSessions(w http.ResponseWriter, r *http.Request) {
-	sessions := h.SessionStore.ListActive()
+	sessions, err := h.Sessions.ListActive(r.Context())
+	if err != nil {
+		slog.Error("list sessions failed", "error", err)
+		writeJSON(w, http.StatusInternalServerError, models.ErrorResponse{Error: "failed to list sessions"})
+		return
+	}
 	if sessions == nil {
 		sessions = []models.Session{}
 	}
@@ -18,9 +24,10 @@ func (h *Handler) ListSessions(w http.ResponseWriter, r *http.Request) {
 	page, pageSize := parsePagination(r)
 	start, end := paginateSlice(len(sessions), page, pageSize)
 
+	totalCount, _ := h.Sessions.Count(r.Context())
 	writeJSON(w, http.StatusOK, models.PaginatedResponse{
 		Data:       sessions[start:end],
-		TotalCount: h.SessionStore.Count(),
+		TotalCount: totalCount,
 		Page:       page,
 		PageSize:   pageSize,
 		HasMore:    end < len(sessions),