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>

Taskfile.yml

@@ -52,14 +52,6 @@ tasks:
     cmds:
       - go build -o bin/tapir ./cmd/tapir
 
-  invite:
-    desc: "Invite a user by email. Usage: task invite EMAIL=user@example.com"
-    requires:
-      vars: [EMAIL]
-    cmds:
-      - task: build
-      - op run --env-file ~/.config/tapir/tapir.env -- ./bin/tapir invite "{{.EMAIL}}"
-
   skills:
     desc: Wire the engineering skills library into this repo (symlinks, gitignored).
     cmds: