feat: WeKan ticketing source

- wekan connector: board id as project key, cards assigned to the
  consultant's WeKan username (assignees with member fallback), archived
  cards skipped, client-side modifiedAt since-filtering, username/password
  login with token reuse (or pre-issued token), case-insensitive identity
- fake-WeKan unit tests: factory validation, test-connection, fetch
  filtering, key listing, token reuse
- admin customer wizard: WeKan option + credential fields
- OpenAPI enum + credential shapes, README identity mapping docs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/domain/customer.go

@@ -8,6 +8,7 @@ const (
 	TicketingJira     TicketingType = "jira"
 	TicketingAzure    TicketingType = "azure_devops"
 	TicketingYouTrack TicketingType = "youtrack"
+	TicketingWekan    TicketingType = "wekan"
 	// TicketingDemo fabricates tickets locally so the whole flow works
 	// offline (§11.11).
 	TicketingDemo TicketingType = "demo"
@@ -15,7 +16,7 @@ const (
 
 func (t TicketingType) Valid() bool {
 	switch t {
-	case TicketingJira, TicketingAzure, TicketingYouTrack, TicketingDemo:
+	case TicketingJira, TicketingAzure, TicketingYouTrack, TicketingWekan, TicketingDemo:
 		return true
 	}
 	return false