v0.29.0: #28 Parts 2+3 — create-RFC offers + contribute-to-pending requests

Extends the v0.26.0 (#28 Part 1) read-time scanner into three buckets in
one pass — active link (Part 1), pending-RFC contribute offer (Part 3),
create-RFC offer (Part 2) — precedence active > pending > candidate. The
backend still emits only structured segments (never HTML), so the surface
stays XSS-safe by construction.

Part 2 — create-RFC offers: a multi-word tag from the #27 taxonomy with no
defining RFC renders, for a create-rights viewer, as an inline "+ create
RFC" affordance that opens the propose modal pre-filled (?propose=<term>;
ProposeModal gained initialTitle). Conservative multi-word gate; broader
heuristics + the Haiku path are deferred.

Part 3 — contribute-to-pending offers: a term matching a super-draft
renders, for a signed-in non-owner, an "ask to contribute" affordance with
the owner's display name. It opens a 3-field request form (who/why/optional
use-case); submitting lands a contribution_requests row (migration 024) and
one actionable §15 notification per owner (new kind
contribution_request_on_pending_rfc, personal-direct). The owner's inbox
shows who/why/use-case inline with Accept/Decline. Accept fires #12's
owner-invite flow with the requester as invitee and echoes a notification
back; decline notifies the requester. Pre-merge idea PRs are out of scope.

New endpoints: GET /api/rfcs/{slug}/contribution-target,
POST /api/rfcs/{slug}/contribution-requests,
.../{id}/accept, .../{id}/decline. The invite issue path was refactored
into one reusable api_invitations.issue_invitation(...) chokepoint shared
by the manual invite endpoint and Part 3's accept.

Tests: 9 new (3 scanner-bucket unit + 6 e2e). Full suite 374 passing.

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

frontend/src/components/Inbox.jsx

@@ -11,6 +11,8 @@
 import { useEffect, useMemo, useState } from 'react'
 import { Link } from 'react-router-dom'
 import {
+  acceptContributionRequest,
+  declineContributionRequest,
   listNotifications,
   markNotificationRead,
   markNotificationsReadByFilter,
@@ -164,7 +166,71 @@ export default function Inbox({ onClose, lastChangeTick }) {
   )
 }
 
+// #28 Part 3: the contribute-request row is the first actionable inbox
+// kind — it renders the requester's who/why/use-case inline and an
+// Accept/Decline pair that fire the owner's decision (accept reuses #12's
+// invite flow on the backend).
+function ContributionRequestRow({ item, onMarkRead }) {
+  const unread = !item.read_at
+  const x = item.extras || {}
+  const [outcome, setOutcome] = useState(null)   // 'accepted' | 'declined'
+  const [busy, setBusy] = useState(false)
+  const [error, setError] = useState(null)
+
+  async function act(accept) {
+    if (busy || outcome) return
+    setBusy(true)
+    setError(null)
+    try {
+      if (accept) await acceptContributionRequest(item.rfc_slug, x.request_id)
+      else await declineContributionRequest(item.rfc_slug, x.request_id)
+      setOutcome(accept ? 'accepted' : 'declined')
+      await onMarkRead(item)
+    } catch (err) {
+      setError(err.message || 'Action failed.')
+    } finally {
+      setBusy(false)
+    }
+  }
+
+  return (
+    <li className={`inbox-row inbox-row-action ${unread ? 'unread' : 'read'}`}>
+      <div className="inbox-row-main">
+        <span className="inbox-unread-dot" aria-hidden />
+        <span className={`inbox-cat cat-${item.category || 'unknown'}`}>{item.category || '·'}</span>
+        <span className="inbox-summary">{item.summary}</span>
+        <span className="inbox-when">{formatWhen(item.created_at)}</span>
+      </div>
+      <div className="inbox-request-detail">
+        {x.who_i_am && <p><strong>Who:</strong> {x.who_i_am}</p>}
+        {x.why && <p><strong>Why:</strong> {x.why}</p>}
+        {x.use_case && <p><strong>Use case:</strong> {x.use_case}</p>}
+      </div>
+      {error && <p className="field-error">{error}</p>}
+      {outcome ? (
+        <p className="inbox-request-outcome muted">
+          {outcome === 'accepted'
+            ? 'Accepted — an invitation has been sent.'
+            : 'Declined.'}
+        </p>
+      ) : (
+        <div className="inbox-request-actions">
+          <button type="button" className="btn-primary" disabled={busy || !x.request_id} onClick={() => act(true)}>
+            Accept
+          </button>
+          <button type="button" className="btn-secondary" disabled={busy || !x.request_id} onClick={() => act(false)}>
+            Decline
+          </button>
+        </div>
+      )}
+    </li>
+  )
+}
+
 function InboxRow({ item, onClick, onMarkRead, onClose }) {
+  if (item.event_kind === 'contribution_request_on_pending_rfc') {
+    return <ContributionRequestRow item={item} onMarkRead={onMarkRead} />
+  }
   const unread = !item.read_at
   const target = deepLink(item)
   const handle = async () => {