feat(api): add feature flag management endpoints

Add FeatureHandler to manage lightweight runtime configuration toggles
stored in the Redis `meta:features` hash. This enables dynamic control
over system behavior (e.g., storage backends) via the admin API.

- Add `GET /admin/features` to list flags
- Add `PUT /admin/features` to update flags
- Update README with feature flag documentation

internal/api/feature_handler.go

@@ -0,0 +1,106 @@
+package api
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	"github.com/redis/go-redis/v9"
+)
+
+const featuresKey = "meta:features"
+
+// FeatureHandler manages lightweight feature flags stored in Redis for DP/CP runtime toggles.
+// Values are stored as plain strings in a Redis hash.
+type FeatureHandler struct {
+	rdb *redis.Client
+}
+
+func NewFeatureHandler(rdb *redis.Client) *FeatureHandler {
+	return &FeatureHandler{rdb: rdb}
+}
+
+// ListFeatures godoc
+// @Summary      List feature flags
+// @Description  Returns all feature flags stored in Redis (meta:features)
+// @Tags         admin
+// @Produce      json
+// @Security     AdminAuth
+// @Success      200      {object}  gin.H
+// @Failure      500      {object}  gin.H
+// @Router       /admin/features [get]
+func (h *FeatureHandler) ListFeatures(c *gin.Context) {
+	if h.rdb == nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "redis not configured"})
+		return
+	}
+	m, err := h.rdb.HGetAll(c.Request.Context(), featuresKey).Result()
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to read features", "details": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"features": m})
+}
+
+type UpdateFeaturesRequest map[string]any
+
+// UpdateFeatures godoc
+// @Summary      Update feature flags
+// @Description  Updates selected feature flags (meta:features). Values are stored as strings.
+// @Tags         admin
+// @Accept       json
+// @Produce      json
+// @Security     AdminAuth
+// @Param        request  body      object  true  "Feature map"
+// @Success      200      {object}  gin.H
+// @Failure      400      {object}  gin.H
+// @Failure      500      {object}  gin.H
+// @Router       /admin/features [put]
+func (h *FeatureHandler) UpdateFeatures(c *gin.Context) {
+	if h.rdb == nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "redis not configured"})
+		return
+	}
+
+	var req UpdateFeaturesRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+
+	updates := make(map[string]string, len(req))
+	for k, v := range req {
+		key := strings.TrimSpace(k)
+		if key == "" {
+			continue
+		}
+		switch vv := v.(type) {
+		case string:
+			updates[key] = strings.TrimSpace(vv)
+		case bool:
+			if vv {
+				updates[key] = "true"
+			} else {
+				updates[key] = "false"
+			}
+		case float64:
+			// JSON numbers decode as float64; keep as integer-looking string when possible.
+			updates[key] = fmt.Sprintf("%v", vv)
+		default:
+			updates[key] = fmt.Sprintf("%v", vv)
+		}
+	}
+
+	if len(updates) == 0 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "no valid feature updates"})
+		return
+	}
+
+	if err := h.rdb.HSet(c.Request.Context(), featuresKey, updates).Err(); err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to update features", "details": err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{"updated": updates})
+}