Add dev container configuration for local dev, Codespaces, and Claude Code

[abkfenris]

Introduce a Dev Containers spec setup under .devcontainer/ so the project can
be opened one-click in VS Code, GitHub Codespaces, or any spec-compatible
editor, with Claude Code preinstalled.

To keep things DRY, service definitions are shared through a new
docker-compose.base.yaml that both the main stack and the dev container extend
via the compose extends: keyword, and the dev container image is a new
devcontainer stage in the existing app/Dockerfile rather than a duplicate
Dockerfile.

• app/Dockerfile: split into base / devcontainer / production stages; the
  production stage stays last so the default build target (test compose and
  make build) is unchanged. The devcontainer stage adds a non-root vscode
  user and interactive tooling.
• docker-compose.base.yaml: shared db, cache, and app (build) definitions.
• docker-compose.yaml: refactored to extend the base; strictly
  behavior-preserving (verified with docker compose config).
• .devcontainer/: devcontainer.json (Claude Code feature, VS Code extensions,
  forwarded ports), docker-compose.yml (extends base, inline dev env, Postgres
  persisted to a docker-data/devcontainer-db bind mount), and an idempotent
  post-create.sh that runs uv sync, migrations, fixture seeding, and default
  superuser creation, skipping seed steps when the DB is already populated.
• .gitignore / Readme.md: ignore the dev container DB dir and document the
  workflow.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01SerCd5rRLrU1TsU8AHCXXQ