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/test_helpers_auth.go

@@ -0,0 +1,25 @@
+package v1
+
+import (
+	"strconv"
+
+	"github.com/gin-gonic/gin"
+)
+
+// handlerTestResponse is a shared response struct for unmarshalling handler test JSON output.
+type handlerTestResponse struct {
+	Success bool        `json:"success"`
+	Data    interface{} `json:"data"`
+}
+
+// mockAuthMiddleware simulates auth middleware for edge-case tests.
+// Sets user_id=1 and extracts account_id from URL param.
+func mockAuthMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Set("user_id", uint(1))
+		if accID, err := strconv.ParseUint(c.Param("account_id"), 10, 32); err == nil {
+			c.Set("account_id", uint(accID))
+		}
+		c.Next()
+	}
+}