#26: optional proposed-use-case field on propose-RFC + propose-PR

Adds an optional "What will you be using this for?" capture as a sibling
to the required justification on both propose surfaces, per roadmap #26.

- propose-RFC modal (ProposeModal): optional textarea below the required
  "Why is this RFC needed?" pitch, labeled "What will you be using this
  RFC for? (optional)".
- propose-PR modal (PRModal): optional textarea below the required
  description, labeled "What will you be using this change for?".
- Backend: ProposeBody / OpenPRBody gain an optional `proposed_use_case`
  (NULL/omitted accepted, no min, 8000-char cap matching the existing
  free-text bound). Persisted to a new canonical side table
  `proposed_use_cases` keyed by PR number, mirrored onto the cache
  columns added by migration 021. Returned on the proposal list/detail,
  RFC detail (by slug), and PR detail endpoints.
- Display: ProposalView, RFCView (main only), and PRView render the
  captured use case with a muted "left blank" treatment when NULL.
- migration 021: nullable `proposed_use_case` on cached_rfcs/cached_prs
  plus the reconcile-proof `proposed_use_cases` truth table.
- New vertical test_proposed_use_case_vertical: persists+returns when
  supplied, accepted as NULL/omitted, for both surfaces.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/PRView.jsx

@@ -220,6 +220,17 @@ export default function PRView({ viewer }) {
               {pr.description && (
                 <p className="pr-description">{pr.description}</p>
               )}
+              {/* #26: the optional ground-truth use case for this change,
+                  captured when the PR was opened. Muted "left blank"
+                  treatment when none was supplied. */}
+              <div className="pr-use-case" style={{ margin: '6px 0', fontSize: 13 }}>
+                <span style={{ fontWeight: 700, color: '#888', textTransform: 'uppercase', letterSpacing: '0.05em', fontSize: 11 }}>
+                  Intended use case:
+                </span>{' '}
+                {pr.proposed_use_case
+                  ? <span style={{ whiteSpace: 'pre-wrap' }}>{pr.proposed_use_case}</span>
+                  : <span style={{ color: '#999', fontStyle: 'italic' }}>left blank</span>}
+              </div>
               {pr.capabilities?.can_edit_text && (
                 <button className="btn-link" onClick={startHeaderEdit}>Edit title & description</button>
               )}