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/pgvector_stub/go.mod

@@ -0,0 +1,3 @@
+module github.com/pgvector/pgvector-go
+
+go 1.24

backend/internal/pgvector_stub/vector.go

@@ -0,0 +1,20 @@
+package pgvector
+
+// Vector represents a vector for similarity search.
+// Stub implementation for building without PostgreSQL/pgvector extension.
+type Vector []float32
+
+// NewVector creates a new vector from a slice of float32.
+func NewVector(slice []float32) Vector {
+	return Vector(slice)
+}
+
+// String returns the string representation of the vector.
+func (v Vector) String() string {
+	return ""
+}
+
+// Dimensions returns the number of dimensions in the vector.
+func (v Vector) Dimensions() int {
+	return len(v)
+}