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/migrations/000044_add_audits_table.up.sql

@@ -0,0 +1,25 @@
+CREATE TABLE IF NOT EXISTS audits (
+  id BIGSERIAL PRIMARY KEY,
+  account_id BIGINT NOT NULL,
+  user_id BIGINT,
+  auditable_type VARCHAR(100) NOT NULL,
+  auditable_id BIGINT NOT NULL,
+  action VARCHAR(50) NOT NULL,
+  audited_changes JSONB DEFAULT '{}'::jsonb,
+  associated_type VARCHAR(100),
+  associated_id BIGINT,
+  username VARCHAR(255),
+  remote_address VARCHAR(100),
+  request_uuid VARCHAR(100),
+  version BIGINT,
+  comment TEXT,
+  user_type VARCHAR(50),
+  created_at TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS idx_audits_account_id ON audits (account_id);
+CREATE INDEX IF NOT EXISTS idx_audits_user_id ON audits (user_id);
+CREATE INDEX IF NOT EXISTS idx_audits_action ON audits (action);
+CREATE INDEX IF NOT EXISTS idx_audits_auditable ON audits (auditable_type, auditable_id);
+CREATE INDEX IF NOT EXISTS idx_audits_associated ON audits (associated_type, associated_id);
+CREATE INDEX IF NOT EXISTS idx_audits_created_at ON audits (created_at);