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.Web/appsettings.json

@@ -42,6 +42,10 @@
       "ApiKey": "25651af3a4829559ef0dfa1758fa3edbf92b6b76"
     }
   },
+  "Columbia": {
+    "ApiKey": "",
+    "BaseUrl": "https://columbiacoatings.com"
+  },
   "SendGrid": {
     "ApiKey": "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",
     "FromEmail": "spouliot@scppowdercoating.com",