Add recipe steps with dynamic form UI

[devin-ai-integration]

Summary

Implements Issue #28 — "As Michael, I want my Recipes to have Steps."

Changes:

• Step model with recipe_id, position (integer, NOT NULL), and body (text, NOT NULL)
• Recipe model updated with has_many :steps (ordered by position, dependent destroy)
• Stimulus steps_controller.js for dynamic add/remove step rows — mirrors the existing ingredients pattern. Step labels auto-renumber (Step 1, Step 2, ...) when rows are added or removed
• Recipe form has a "Steps" section below ingredients with + Add Step button, text area per step, and × remove button
• Recipe show page displays steps as a numbered list below ingredients
• Controller handles step saving with position auto-assigned from form order. Blank steps and destroyed rows are skipped
• Tests: 100 tests, 306 assertions — model validations, CRUD with steps, destroy cascade, position ordering

Review & Testing Checklist for Human

• Create a new recipe with 2-3 steps — verify steps save and display correctly on show page
• Edit the recipe — verify existing steps load in the form, add/remove steps, save and confirm changes
• Verify step numbers renumber dynamically when removing a step from the middle
• Delete a recipe with steps — confirm steps are also deleted (no orphans)
• Create a recipe with only ingredients (no steps) — verify the steps section doesn't appear on show page

Notes

• Position is auto-assigned from the visual order of steps in the form — users never set it manually
• Steps are destroyed and re-created on each save (same pattern as ingredients) to keep position assignment simple
• This PR does NOT add step parsing from URLs — that's Issue #29

Link to Devin session: https://app.devin.ai/sessions/06cbafdef49a46e88741224f05c8f84e
Requested by: @utricularian

[devin-ai-integration]

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