Social composer: send now and rename messy account names (#6)

## Summary
Closes #5.
- Composer can **Send now** without a schedule time (Schedule and Save draft still available)
- Account names wrap and show the platform id so a bad OAuth label is still readable
- Social accounts page can rename the display name; re-auth keeps a custom name instead of overwriting it with a placeholder
- Worker polls `dispatch_due` every 15s so scheduled campaigns and social posts fire without a manual command; local `docker compose up` now starts the worker

## Test plan
- [ ] Open Social → Compose: confirm **Send now**, **Schedule**, and **Save draft** buttons (no combined Save / publish)
- [ ] Send now with a connected account and caption → post queues immediately, no datetime required
- [ ] Schedule with a datetime → post is scheduled; Schedule with a blank time → validation error
- [ ] Composer account list shows full name plus external id, wrapping if the name is long
- [ ] Social accounts → rename a messy label → new name shows in composer without reconnecting
- [ ] Re-authorize an account that was renamed → custom name is kept
- [ ] Rebuild and start compose (`docker compose up --build`) so the **worker** service is running
- [ ] Schedule a social post ~2 minutes ahead → after due time, worker logs `Enqueued N due item(s).` and the post leaves Scheduled (published or failed)
- [ ] `docker compose logs -f worker` shows `Polling due scheduled work every 15s.`

Reviewed-on: #6

README.md

@@ -25,13 +25,14 @@ uv sync
 cd site
 uv run python manage.py migrate
 uv run python manage.py runserver
-# optional: docker compose up   # web + postgres (hot reload)
-# optional worker: docker compose --profile worker up worker
+# optional: docker compose up   # web + postgres + worker (hot reload)
 ```
 
 Docker compose mounts `./site` into the container and runs Django `runserver` when
-`DJANGO_ENV=dev` (the default). Edit Python/templates → auto-reload; no image rebuild.
-Rebuild only when Dockerfile / deps (`pyproject.toml`, `uv.lock`) change:
+`DJANGO_ENV=dev` (the default). The **worker** service polls `dispatch_due` every 15s so
+scheduled campaigns and social posts fire without a manual command. Edit Python/templates
+→ auto-reload; no image rebuild.
+Rebuild only when Dockerfile, entrypoint scripts, or deps (`pyproject.toml`, `uv.lock`) change:
 
 ```bash
 docker compose up --build