Install palace-manager editable in the container (PP-4076) (#3245)

## Description

Drop `--no-editable` from the final `uv sync` in `docker/Dockerfile` so
`palace-manager` is installed editably in the container — same behavior
we had under Poetry.

## Motivation and Context

Found while checking the uv changes that rolled out to Minotaur last
night. The Poetry → uv migration in #3221 switched the in-container
install from editable (Poetry's default) to non-editable (`uv sync
--no-editable`). That change quietly broke
`palace.manager.scripts.startup`'s discovery of the `startup_tasks/`
directory:

```
Startup tasks directory /var/www/circulation/env/lib/python3.12/startup_tasks does not exist; skipping.
```

The script resolves `STARTUP_TASKS_DIR` via `Path(__file__).parents[4]`,
which assumes `startup.py` is at
`${root}/src/palace/manager/scripts/startup.py` (editable install).
Under `--no-editable`, `startup.py` lives inside `site-packages` and
`parents[4]` resolves to `env/lib/python3.12/` instead of the project
root, so `startup_tasks/` is never found.

Restoring the editable install fixes the immediate breakage and matches
the previous behavior. It also has the minor side benefit of making
in-container debugging easier — you can edit files in place without
rebuilding the image.

## Future work

Eventually we'd like to build `palace-manager` as a proper non-editable
package that can be published the same way `palace-util` and
`palace-opds` will be. Several path-resolution issues need to be sorted
out first (alembic migrations, startup scripts, and similar file-layout
assumptions) before that's feasible. For now, editable install is the
simpler and safer choice.

## How Has This Been Tested?

- Built the image locally and confirmed `startup.py` now resolves
`STARTUP_TASKS_DIR` to `/var/www/circulation/startup_tasks`.
- Existing startup task
(`2026_02_17_force_re_harvest_all_opds_for_distributors.py`) is picked
up.

## Checklist

- [x] I have updated the documentation accordingly.
- [x] All new and existing tests passed.

Author: jonathangreen

docker/Dockerfile

@@ -27,10 +27,8 @@ RUN uv sync --frozen --no-dev --no-install-workspace
 
 COPY --chown=palace:palace . /var/www/circulation
 
-# Install the project itself as a non-editable install. Omitting
-# --no-install-project (and passing --no-editable) is the recommended
-# pattern for production Docker builds with uv.
-RUN uv sync --frozen --no-dev --no-editable
+# Install the project itself as an editable install.
+RUN uv sync --frozen --no-dev
 
 ###############################################################################
 ## Circ-exec Image