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/configs/fluentd.conf

@@ -0,0 +1,65 @@
+# GoChat Fluentd Configuration
+# Reference: Chatwoot production uses structured JSON logging
+# This config collects logs from gochat containers and forwards to outputs
+
+<source>
+  @type tail
+  path /var/log/gochat/*.log
+  pos_file /var/log/fluentd/gochat.log.pos
+  tag gochat.app
+  <parse>
+    @type json
+    time_key timestamp
+    time_format %Y-%m-%dT%H:%M:%S.%NZ
+    keep_time_key true
+  </parse>
+</source>
+
+<source>
+  @type tail
+  path /var/log/gochat/worker.log
+  pos_file /var/log/fluentd/gochat-worker.log.pos
+  tag gochat.worker
+  <parse>
+    @type json
+    time_key timestamp
+    time_format %Y-%m-%dT%H:%M:%S.%NZ
+    keep_time_key true
+  </parse>
+</source>
+
+# Docker container logs via systemd/journald
+<source>
+  @type systemd
+  filters [{ "_COMM": "docker" }]
+  tag docker
+</source>
+
+# ---- Outputs ----
+
+# Console output for development
+<match gochat.**>
+  @type stdout
+</match>
+
+# Production: send to Elasticsearch or Loki
+# Uncomment based on your backend:
+#
+# <match gochat.**>
+#   @type elasticsearch
+#   host elasticsearch
+#   port 9200
+#   logstash_format true
+#   logstash_prefix gochat
+#   <buffer>
+#     @type file
+#     path /var/log/fluentd/buffers/gochat
+#     flush_interval 5s
+#   </buffer>
+# </match>
+#
+# <match gochat.**>
+#   @type loki
+#   url http://loki:3100
+#   labels { "app": "gochat", "environment": "production" }
+# </match>