Add Columbia Coatings API client, DTOs, and sync settings

Phase 1b of the Columbia Coatings integration: the typed read client and
its configuration, ahead of the sync/mapper service.

- ColumbiaProductDtos: wire-shape models for GET /products. tiered_pricing
  is captured as JsonElement because the API returns it as an object on
  simple products but an empty array on variable ones — binding it raw
  avoids a deserialization throw; the mapper interprets it.
- IColumbiaCoatingsApiClient / ColumbiaCoatingsApiClient: pages the catalog
  via GET /products (NOT the export download_url, which is Cloudflare-blocked
  for server clients). Sends X-API-Key from config, honors 429/Retry-After,
  and THROWS on any page failure so a partial pull can never be mistaken for
  the full catalog (protects the later discontinuation sweep).
- ColumbiaIntegrationConstants: single home for config keys, setting keys,
  and the derived Source/manufacturer/category values.
- Config: Columbia:ApiKey (blank — secret supplied per environment) and
  Columbia:BaseUrl in appsettings.
- SeedColumbiaSyncSettings migration: seeds SuperAdmin-managed platform
  settings ColumbiaSyncEnabled (off by default), ColumbiaSyncIntervalDays
  (7), and last-sync tracking, under a new "Integrations" group.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/PowderCoating.Application/Interfaces/IColumbiaCoatingsApiClient.cs

@@ -0,0 +1,35 @@
+using PowderCoating.Application.DTOs.Columbia;
+
+namespace PowderCoating.Application.Interfaces;
+
+/// <summary>
+/// Typed client for the Columbia Coatings product catalog API (<c>/wp-json/cca/v1</c>).
+/// Read-only: lists products via the paged <c>GET /products</c> endpoint.
+/// <para>
+/// We deliberately page <c>/products</c> rather than using the bulk <c>export.json</c> download:
+/// the export returns a temporary <c>download_url</c> to a static file under <c>/wp-content/uploads</c>,
+/// which sits behind Cloudflare bot protection and 403s for non-browser clients. The
+/// <c>/wp-json</c> API routes are allowlisted via the API key, so paging is the only path that
+/// works reliably from a server.
+/// </para>
+/// </summary>
+public interface IColumbiaCoatingsApiClient
+{
+    /// <summary>
+    /// True when an API key is configured (<c>Columbia:ApiKey</c>). When false, callers should
+    /// skip the sync entirely rather than issue unauthenticated requests.
+    /// </summary>
+    bool IsConfigured { get; }
+
+    /// <summary>
+    /// Retrieves a single page of products. <paramref name="perPage"/> is capped at 100 by the API.
+    /// </summary>
+    Task<ColumbiaProductsResponse> GetProductsPageAsync(int page, int perPage, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Pages through the entire catalog and returns every product. Honors rate limiting
+    /// (429 / Retry-After). THROWS if any page fails after retries — callers must treat an
+    /// exception as "incomplete pull" and NOT run discontinuation logic against a partial set.
+    /// </summary>
+    Task<IReadOnlyList<ColumbiaProduct>> GetAllProductsAsync(CancellationToken cancellationToken = default);
+}