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/model/inbox_limit.go

@@ -0,0 +1,21 @@
+package model
+
+// InboxLimit represents a constraint/limit applied to an inbox (e.g. max conversation count).
+// Reference: Chatwoot app/models/inbox_limit.rb + InboxLimitsController
+// Inbox limits control operational constraints for specific inboxes — only create/update/destroy
+// (no index or show endpoints in Chatwoot).
+//
+// Chatwoot compatibility notes:
+//   - Routes: POST/PATCH/DELETE under inbox scope
+//   - type field: e.g. "conversation_count", "message_count" — identifies which dimension is limited
+//   - value field: the numeric limit threshold
+type InboxLimit struct {
+	Base
+	InboxID uint   `gorm:"not null;index" json:"inbox_id"`
+	Type    string `gorm:"size:100;not null" json:"type"`
+	Value   int    `gorm:"not null" json:"value"`
+	Inbox   Inbox  `gorm:"foreignKey:InboxID" json:"inbox,omitempty"`
+}
+
+// TableName returns the database table name for InboxLimit.
+func (InboxLimit) TableName() string { return "inbox_limits" }