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.Infrastructure/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -7234,7 +7234,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 1,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 6, 17, 14, 21, 27, 126, DateTimeKind.Utc).AddTicks(6644),
+                            CreatedAt = new DateTime(2026, 6, 17, 14, 44, 40, 147, DateTimeKind.Utc).AddTicks(5997),
                             Description = "Standard pricing for regular customers",
                             DiscountPercent = 0m,
                             IsActive = true,
@@ -7245,7 +7245,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 2,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 6, 17, 14, 21, 27, 126, DateTimeKind.Utc).AddTicks(6651),
+                            CreatedAt = new DateTime(2026, 6, 17, 14, 44, 40, 147, DateTimeKind.Utc).AddTicks(6002),
                             Description = "5% discount for preferred customers",
                             DiscountPercent = 5m,
                             IsActive = true,
@@ -7256,7 +7256,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 3,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 6, 17, 14, 21, 27, 126, DateTimeKind.Utc).AddTicks(6652),
+                            CreatedAt = new DateTime(2026, 6, 17, 14, 44, 40, 147, DateTimeKind.Utc).AddTicks(6003),
                             Description = "10% discount for premium customers",
                             DiscountPercent = 10m,
                             IsActive = true,