Wire competitor models, task type, usage count fix, and Competitors page handler

- Register Competitor + CompetitorQuote in AllModels()
- Add TaskTypeCompetitorImport task type constant
- CountUsage now checks pricelist_id, warehouse_pricelist_id, competitor_pricelist_id
- WebHandler: load competitors.html template, add Competitors() page handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/models/models.go

@@ -23,6 +23,8 @@ func AllModels() []interface{} {
 		&PartnumberBook{},
 		&PartnumberBookItem{},
 		&VendorPartnumberSeen{},
+		&Competitor{},
+		&CompetitorQuote{},
 	}
 }