feat: add mix hammer.install Igniter task

[epinault]

Summary

• Adds an Igniter installer so mix igniter.install hammer (or mix hammer.install on a project that already has Hammer) generates a MyApp.RateLimit module and adds it to the application's supervision tree.
• --backend / -b accepts ets (default), atomic or redis. Because the Redis backend lives in the separate hammer_backend_redis package, --backend redis also adds that dependency and generates use Hammer, backend: Hammer.Redis with a url: child spec.
• Igniter is an optional dependency: without it, mix hammer.install prints instructions to install Igniter (following Igniter's own optional-dependency template). No runtime impact for existing users.

This is part 1 of #157 (installer). It deliberately does not attempt the "patch my controller functions" half of that issue: Hammer has no global/per-route config — limits are defined at each call site via hit(key, scale, limit) — so a patcher can't know the key derivation, limits, or 429 handling a given app needs. The README addition points users at the Tutorial for that.

Changes

• lib/mix/tasks/hammer.install.ex - the Mix.Tasks.Hammer.Install task (+ .Docs module, non-Igniter fallback)
• test/mix/tasks/hammer.install_test.exs - Igniter.Test coverage: all three backends, dep added / not duplicated for redis, unknown backend reported as an issue, idempotent re-run
• mix.exs / mix.lock - {:igniter, "~> 0.8", optional: true}
• README.md - "Installing with Igniter" subsection
• CHANGELOG.md - Unreleased entry

Generated output for --backend ets in an app named MyApp:

# lib/my_app/rate_limit.ex
defmodule MyApp.RateLimit do
  @moduledoc """
  Rate limiter for MyApp, backed by `:ets`.
  ...
  """

  use Hammer, backend: :ets
end

# lib/my_app/application.ex
children = [
  {MyApp.RateLimit, [clean_period: :timer.minutes(1)]},
  ...
]

Test Plan

• mix test --warnings-as-errors --cover — 142 tests, 0 failures, 93.2% total coverage (task at 100%)
• mix credo --strict, mix format --check-formatted, mix deps.unlock --check-unused
• End-to-end in a fresh mix new --sup project with {:hammer, path: ...}: mix hammer.install generates the module + child spec, and MyApp.RateLimit.hit/3 returns {:allow, 1}, {:allow, 2}, {:deny, _} with limit 2
• End-to-end mix hammer.install --backend redis: adds {:hammer_backend_redis, "~> 7.0"}, generated module compiles against hammer_backend_redis 7.1.1

Ticket

Refs: #157

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🤖 Generated with Claude Code by Emmanuel Pinault

https://claude.ai/code/session_01JckHuT8GcW8Gzsai7vcpvT