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/assignment_policy.go

@@ -0,0 +1,42 @@
+package model
+
+import (
+	"time"
+)
+
+// AssignmentPolicy represents a conversation assignment policy for an account.
+// Reference: Chatwoot app/models/assignment_policy.rb + M5 spec
+type AssignmentPolicy struct {
+	ID                   uint      `gorm:"primaryKey" json:"id"`
+	AccountID            uint      `gorm:"not null;index;uniqueIndex:idx_account_policy_name" json:"account_id"`
+	Name                 string    `gorm:"size:255;not null;uniqueIndex:idx_account_policy_name" json:"name"`
+	Description          string    `gorm:"type:text" json:"description"`
+	AssignmentOrder      int       `gorm:"default:0" json:"assignment_order"`     // 0=round_robin, 1=balanced (enterprise)
+	ConversationPriority int       `gorm:"default:0" json:"conversation_priority"` // 0=earliest_created, 1=longest_waiting
+	FairDistributionLimit  int     `gorm:"default:100" json:"fair_distribution_limit"`
+	FairDistributionWindow int     `gorm:"default:3600" json:"fair_distribution_window"`
+	Enabled              bool      `gorm:"default:true" json:"enabled"`
+	CreatedAt            time.Time `gorm:"autoCreateTime" json:"created_at"`
+	UpdatedAt            time.Time `gorm:"autoUpdateTime" json:"updated_at"`
+
+	Account                Account                `gorm:"foreignKey:AccountID" json:"account,omitempty"`
+	InboxAssignmentPolicies []InboxAssignmentPolicy `gorm:"foreignKey:AssignmentPolicyID" json:"inbox_assignment_policies,omitempty"`
+}
+
+func (AssignmentPolicy) TableName() string { return "assignment_policies" }
+
+// InboxAssignmentPolicy represents the join between Inbox and AssignmentPolicy.
+// Each Inbox can only be associated with one AssignmentPolicy.
+// Reference: Chatwoot app/models/inbox_assignment_policy.rb + M5 spec
+type InboxAssignmentPolicy struct {
+	ID                uint      `gorm:"primaryKey" json:"id"`
+	InboxID           uint      `gorm:"not null;uniqueIndex" json:"inbox_id"`
+	AssignmentPolicyID uint     `gorm:"not null;index" json:"assignment_policy_id"`
+	CreatedAt         time.Time `gorm:"autoCreateTime" json:"created_at"`
+	UpdatedAt         time.Time `gorm:"autoUpdateTime" json:"updated_at"`
+
+	Inbox             Inbox            `gorm:"foreignKey:InboxID" json:"inbox,omitempty"`
+	AssignmentPolicy  AssignmentPolicy `gorm:"foreignKey:AssignmentPolicyID" json:"assignment_policy,omitempty"`
+}
+
+func (InboxAssignmentPolicy) TableName() string { return "inbox_assignment_policies" }