refactor: remove Dex local-password invite provisioning (ADR-019)

Authentik owns invites now (infra ADR-0001). Delete adapters/dex, the
/invite set-password UI, the tapir invite CLI, the InvitationStore/
DexPasswordCreator ports + App wiring, the invite Templ pages, and the
invite Taskfile target. New users are invited via Authentik, log in via
OIDC, and hit the existing /register gate. invitations table (mig 009)
left in place (append-only; harmless). task check green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/flash.go

@@ -11,12 +11,11 @@ const flashCookie = "tapir_flash"
 // Flash codes. Kept small and stable — the message + severity live in
 // flashMessages (view.go), not here, so the cookie never carries free text.
 const (
-	flashConnected      = "connected"
-	flashConnectFailed  = "connect_failed"
-	flashDisconnected   = "disconnected"
-	flashDeleted        = "deleted"
-	flashRegistered     = "registered"
-	flashAccountCreated = "account_created"
+	flashConnected     = "connected"
+	flashConnectFailed = "connect_failed"
+	flashDisconnected  = "disconnected"
+	flashDeleted       = "deleted"
+	flashRegistered    = "registered"
 )
 
 // flashMaxAge bounds how long an unread flash lingers (seconds). Long enough to