feat: show backfill instructions in UI when script is selected

[miguellarranaga-factorial]

Summary

• Add self.instructions class method to DataDrip::Backfill (returns nil by default, backward-compatible)
• Return instructions in the backfill_options JSON response alongside html
• Render the instructions in a styled info box in the new backfill run form when a script is selected
• Lightweight rich text formatter supporting # headers, **bold**, `inline code`, - bullet lists, and triple-backtick code blocks
• Add commented self.instructions placeholder to the generator template
• Update both test backfills with example instructions showcasing all formatting features

Motivation

When selecting a backfill script in the DataDrip UI, there was no way for script authors to communicate what the script does or how to fill in the options. This adds a standard, clean mechanism for that.

Backfill authors can now override self.instructions in their class using a simple markup syntax:

class MyBackfill < DataDrip::Backfill
  def self.instructions
    <<~INSTRUCTIONS
      # My Backfill
      Sets the default role on employees missing one.

      ## Options
      - `department`: limit to a **specific** department

      ## Finding the department ID
      ```
      SELECT id FROM departments WHERE name = 'Engineering';
      ```
    INSTRUCTIONS
  end
end

Supported formatting

Syntax	Renders as
# Title	Large heading
## Section	Blue uppercase section header
### Subsection	Subtle subheader
**text**	Bold
`code`	Inline code pill
- item or * item	Bullet list
Triple backticks	Code block with dark background

The instructions are HTML-escaped before formatting to prevent XSS.

Review feedback addressed

• Renamed description → instructions across the API, controller, JSON response, generator template, specs and test backfills.
• Heredocs are now used for the example instructions in the test backfills (per review).
• Markdown formatter moved out of the inline <script> into app/javascript/data_drip/markdown.js, and the form is now driven by a Stimulus controller (backfill_form_controller.js) — addressing the "leave it in the JS folder" suggestion. We kept our ~60-line renderer rather than pulling in marked: this is an importmap project (no npm/bundler) so marked isn't already available, and a full CommonMark parser isn't a tiny dependency for the small subset we render.
• Inline styles kept on purpose. The engine's Tailwind build can't generate utilities for class names it never sees in a scanned template, and the instructions HTML is injected dynamically — so Tailwind utility classes wouldn't be compiled into tailwind.css. See the inline comment on the PR for details.

Test plan

• spec/lib/data_drip/backfill_spec.rb — tests for .instructions
• spec/controllers/data_drip/backfill_runs_controller_spec.rb — tests for the backfill_options instructions response
• Full suite green (bundle exec rspec) — 113 examples, 0 failures

[miguellarranaga-factorial]

Closing in favor of #21 — this branch (feat/backfill-description) has been merged into #21's branch (feat/add_description), where the instructions feature now ships alongside the description DSL and backfills catalog. See the 'Merged in #20' section of #21. Full combined suite is green (128 examples, 0 failures).