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/scripts/coverage_report.sh

@@ -0,0 +1,94 @@
+#!/bin/bash
+# Coverage rate report generator for gochat project
+# Generates per-package and overall coverage metrics aligned with Chatwoot test patterns.
+set -e
+
+cd "$(dirname "$0")/.."
+export GOPROXY=https://goproxy.cn,direct
+export GONOSUMCHECK=*
+export GONOSUMDB=*
+export GOFLAGS=-mod=mod
+
+REPORT_DIR="coverage_reports"
+mkdir -p "$REPORT_DIR"
+
+echo "=== GoChat Test Coverage Report ==="
+echo "Generated: $(date)"
+echo ""
+
+# Packages to test (only those that compile cleanly)
+TESTABLE_PKGS=(
+    "./internal/model"
+    "./internal/model/channel"
+    "./internal/auth"
+    "./internal/config"
+    "./pkg/testutil"
+    "./pkg/crypto"
+    "./internal/repository"
+    "./internal/pubsub"
+)
+
+TOTAL_COVERAGE=0
+PKG_COUNT=0
+PASS_COUNT=0
+FAIL_COUNT=0
+TOTAL_TESTS=0
+TOTAL_ASSERTIONS=0
+
+echo "| Package | Tests | Coverage | Status |"
+echo "|---------|-------|----------|--------|"
+
+for pkg in "${TESTABLE_PKGS[@]}"; do
+    COVERAGE_FILE="$REPORT_DIR/coverage_$(echo $pkg | tr '/' '_').out"
+    
+    # Run tests with coverage
+    RESULT=$(go test -v -cover -coverprofile="$COVERAGE_FILE" "$pkg" 2>&1)
+    EXIT_CODE=$?
+    
+    # Extract coverage percentage
+    COVERAGE=$(echo "$RESULT" | grep "coverage:" | awk '{print $2}' | sed 's/%//')
+    if [ -z "$COVERAGE" ]; then
+        COVERAGE="0.0"
+    fi
+    
+    # Count tests
+    TEST_COUNT=$(echo "$RESULT" | grep -c "^--- PASS\|^--- FAIL" || echo 0)
+    
+    # Determine status
+    if [ $EXIT_CODE -eq 0 ]; then
+        STATUS="PASS"
+        PASS_COUNT=$((PASS_COUNT+1))
+    else
+        STATUS="FAIL"
+        FAIL_COUNT=$((FAIL_COUNT+1))
+    fi
+    
+    echo "| $pkg | $TEST_COUNT | ${COVERAGE}% | $STATUS |"
+    
+    TOTAL_TESTS=$((TOTAL_TESTS+TEST_COUNT))
+    TOTAL_COVERAGE=$(echo "$TOTAL_COVERAGE + $COVERAGE" | bc)
+    PKG_COUNT=$((PKG_COUNT+1))
+done
+
+echo ""
+echo "=== Summary ==="
+echo "Total packages tested: $PKG_COUNT"
+echo "Passed: $PASS_COUNT, Failed: $FAIL_COUNT"
+echo "Total test functions: $TOTAL_TESTS"
+
+if [ $PKG_COUNT -gt 0 ]; then
+    AVG_COVERAGE=$(echo "$TOTAL_COVERAGE / $PKG_COUNT" | bc -l)
+    echo "Average coverage: ${AVG_COVERAGE}%"
+fi
+
+echo ""
+echo "=== Test File Inventory ==="
+find . -name "*_test.go" -not -path "./vendor/*" | while read f; do
+    LINES=$(wc -l < "$f")
+    FUNCTIONS=$(grep -c "func Test" "$f" 2>/dev/null || echo 0)
+    echo "  $f: ${LINES} lines, ${FUNCTIONS} test functions"
+done
+
+echo ""
+echo "Coverage profiles saved to $REPORT_DIR/"
+echo "To view HTML coverage: go tool cover -html=$REPORT_DIR/coverage_*.out"