feat(slabs): S2 CRUD + optimistic concurrency + soft delete + slab list UI

- backend/slabs.py: create (calibrated, client-derived stored as sent), list (excl
  soft-deleted, most recent), get, update (single compare-and-set on server_rev,
  409 revision_conflict / 409 duplicate_sku / 422 stale_field, status server-derived),
  delete (soft, published rejected). Explicit column allowlists, parameterized SQL,
  JSON array fields, commits.
- backend/app.py: wire slabs router.
- frontend: src/api/slabs.ts client + create/slabs/SlabList.svelte + dev toggle.
- tests: 9 slabs tests (create/dup/list/get/update/rev-conflict/dup-sku/soft-delete/
  published-delete) with full taxonomy seeding.

Verified: pytest 23 passed; check_requirements + check_coverage exit 0; npm build ok.

frontend/src/create/slabs/SlabList.svelte

@@ -0,0 +1,75 @@
+<script lang="ts">
+  import { onMount } from 'svelte';
+  import { listSlabs, type Slab } from '../../api/slabs';
+
+  let slabs: Slab[] = [];
+  let loading = true;
+  let error = '';
+
+  async function load() {
+    loading = true;
+    error = '';
+    try {
+      slabs = await listSlabs();
+    } catch (e) {
+      error = e instanceof Error ? e.message : 'Failed to load slabs';
+    }
+    loading = false;
+  }
+
+  onMount(() => {
+    void load();
+  });
+</script>
+
+<div class="slab-list">
+  <div class="head">
+    <h2>Slabs</h2>
+    <button on:click={() => void load()}>Refresh</button>
+  </div>
+  {#if loading}
+    <p>Loading…</p>
+  {:else if error}
+    <p class="err">{error}</p>
+  {:else if slabs.length === 0}
+    <p class="empty">No slabs yet.</p>
+  {:else}
+    <table>
+      <thead>
+        <tr>
+          <th>SKU</th>
+          <th>Size (L×T)</th>
+          <th>Avg W</th>
+          <th>sqft</th>
+          <th>Price</th>
+          <th>Status</th>
+          <th>Updated</th>
+        </tr>
+      </thead>
+      <tbody>
+        {#each slabs as s (s.id)}
+          <tr>
+            <td>{s.sku}</td>
+            <td>{s.length_in}″ × {s.thickness_in}″</td>
+            <td>{s.width_avg_in}″</td>
+            <td>{s.sqft}</td>
+            <td>${s.price.toFixed(2)}</td>
+            <td><span class="status" class:ready={s.status === 'ready'}>{s.status}</span></td>
+            <td>{s.updated_at?.replace('T', ' ').slice(0, 16)}</td>
+          </tr>
+        {/each}
+      </tbody>
+    </table>
+  {/if}
+</div>
+
+<style>
+  .head { display: flex; justify-content: space-between; align-items: center; }
+  .slab-list { max-width: 720px; color: #eee; }
+  table { width: 100%; border-collapse: collapse; margin-top: 12px; }
+  th, td { text-align: left; padding: 6px 8px; border-bottom: 1px solid #333; font-size: 0.9rem; }
+  .status { text-transform: capitalize; }
+  .status.ready { color: #7fd37f; }
+  .err { color: #e88; }
+  .empty { color: #888; }
+</style>