Reorganize repo: backend/, deploy/, docs/ layout + AGENTS.md

Restructure the monorepo into clear top-level directories:
- backend/: Go module root (cmd, internal, pkg, configs, migrations,
  docs/swagger, scripts, tests, go.mod, Makefile, .air.toml)
- deploy/: Docker (Dockerfile, docker-compose*), quickstart, fluentd
- docs/: project documentation + reports/ (moved from repo root)
- AGENTS.md: new AI coding-agent guide at repo root

Update all references to the new layout:
- Dockerfile: COPY backend/go.mod, COPY backend/ (context = repo root)
- docker-compose files: context ../.., dockerfile deploy/docker/Dockerfile,
  env_file ../../.env, volume mounts ../../backend:/app
- deploy/quickstart/compose.yaml: dockerfile deploy/docker/Dockerfile
- CI: working-directory: backend for go commands, file deploy/docker/Dockerfile,
  coverage path backend/coverage.out, health_check backend/scripts/
- backend/Makefile: docker target uses -f ../deploy/docker/Dockerfile ../
- README: architecture tree, quickstart, config paths updated

Move root stray scripts (rename_models.*, run_m11_tests.sh, verify_build.sh,
gorm_bool_main.go) to backend/scripts/legacy/. All moves via git mv to
preserve history. Build, vet, SQLite tests, and docker compose config verified.

backend/internal/handler/api/v1/agent_bulk_handler.go

@@ -0,0 +1,100 @@
+package v1
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"github.com/gochat/gochat/internal/service"
+	"github.com/gochat/gochat/pkg/response"
+)
+
+// AgentBulkHandler handles bulk assignment/unassignment of conversations to agents.
+// Reference: Chatwoot app/controllers/api/v1/accounts/agents_controller.rb #bulk_assign, #bulk_unassign
+type AgentBulkHandler struct {
+	conversationSvc *service.ConversationService
+}
+
+// NewAgentBulkHandler creates a new AgentBulkHandler.
+func NewAgentBulkHandler(conversationSvc *service.ConversationService) *AgentBulkHandler {
+	return &AgentBulkHandler{conversationSvc: conversationSvc}
+}
+
+// BulkAssignRequest is the DTO for bulk assigning conversations to an agent.
+type BulkAssignRequest struct {
+	ConversationIDs []uint `json:"conversation_ids" binding:"required,min=1"`
+	AgentID         uint   `json:"agent_id" binding:"required"`
+}
+
+// BulkAssign assigns multiple conversations to a single agent.
+// POST /api/v1/accounts/:account_id/agents/bulk_assign
+func (h *AgentBulkHandler) BulkAssign(c *gin.Context) {
+	accountID, err := parseUintParam(c, "account_id")
+	if err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid account_id")
+		return
+	}
+
+	var req BulkAssignRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrValidation, err.Error())
+		return
+	}
+
+	results := make([]map[string]interface{}, 0, len(req.ConversationIDs))
+	for _, convID := range req.ConversationIDs {
+		_, svcErr := h.conversationSvc.AssignAgent(c.Request.Context(), accountID, convID, req.AgentID)
+		if svcErr != nil {
+			results = append(results, map[string]interface{}{
+				"conversation_id": convID,
+				"error":           svcErr.Error(),
+			})
+		} else {
+			results = append(results, map[string]interface{}{
+				"conversation_id": convID,
+				"status":          "assigned",
+			})
+		}
+	}
+
+	response.OK(c, results)
+}
+
+// BulkUnassignRequest is the DTO for bulk unassigning conversations from agents.
+type BulkUnassignRequest struct {
+	ConversationIDs []uint `json:"conversation_ids" binding:"required,min=1"`
+}
+
+// BulkUnassign removes agent assignment from multiple conversations.
+// POST /api/v1/accounts/:account_id/agents/bulk_unassign
+func (h *AgentBulkHandler) BulkUnassign(c *gin.Context) {
+	accountID, err := parseUintParam(c, "account_id")
+	if err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid account_id")
+		return
+	}
+
+	var req BulkUnassignRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrValidation, err.Error())
+		return
+	}
+
+	results := make([]map[string]interface{}, 0, len(req.ConversationIDs))
+	for _, convID := range req.ConversationIDs {
+		_, svcErr := h.conversationSvc.UnassignAgent(c.Request.Context(), accountID, convID)
+		if svcErr != nil {
+			results = append(results, map[string]interface{}{
+				"conversation_id": convID,
+				"error":           svcErr.Error(),
+			})
+		} else {
+			results = append(results, map[string]interface{}{
+				"conversation_id": convID,
+				"status":          "unassigned",
+			})
+		}
+	}
+
+	response.OK(c, results)
+}