Phase 1: convert cosmic-fastapi into a Copier template

[tomasanchez]

Phase 1 — Convert cosmic-fastapi into a Copier template

First slice of the conversion epic #24. The repository is now a working Copier template, validated end-to-end by a bake-and-test that renders the template and runs the generated project's full quality gate.

What changed

• ADR 0015 records the Copier-over-cookiecutter decision (native copier update, Jinja in contents+paths, typed questions).
• copier.yml defines the questions: project_name, project_slug, package_name, identity (author_*, github_owner, license), python_version, database_url, and include_user_example; a post-copy uv lock task.
• The generated project moved under template/ (Copier _subdirectory); the package was renamed template → {{ package_name }} and identity/Python-version/license/DB defaults were templatized across 62 .jinja files (pyproject, Dockerfile, compose, CI, settings, migrations, README, LICENSE). GitHub Actions ${{ }} expressions are wrapped in {% raw %}.
• Root template-authoring project with a bake-and-test harness (tests/test_bake.py) and Template CI that render the template and assert the generated suite passes Ruff + Pyrefly + pytest at 100% coverage.
• Author-facing root README.md, .gitignore, and .pre-commit-config.yaml.

Validation

Independently baked with default/sample answers → generated project: uv sync ✅, ruff check ✅, pyrefly check ✅, 53 tests, 100% coverage ✅.

Deferred (tracked, not in this PR)

• feat(template): optional feature flags and post-generation tasks #21 — include_user_example false-path (Jinja guards + _exclude); the User slice is always included for now.
• docs(template): author-facing vs generated-project docs allocation #23 — author-vs-generated doc split; ADRs/AGENTS.md keep some Cosmic FastAPI/src/template prose, and ADR 0015 currently ships into generated projects.

Closes #20
Refs #22, #24

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