Run seeds only once (#475)

## Description of the implemented changes
The changes were:
- [ ] Bugfixing
- [x] New Feature
- [ ] Breaking Change
- [x] Refactoring

**Seeds run only on first startup.** On every application start (e.g. `just run`, Docker entrypoint), seed scripts are still invoked, but they exit immediately when the admin user already exists. This avoids duplicate seed data (e.g. join requests), keeps startup fast after the first run, and works the same in dev and production.

## What has been changed?

- **`lib/mv/release.ex`**
  - Added `bootstrap_seeds_applied?/0`: returns whether the admin user (from `ADMIN_EMAIL` or default `admin@localhost`) exists. We check the admin *user*, not the Admin *role*, so we do not skip when only migrations have run (migrations can create the Admin role for the system actor).
  - `run_seeds/0`: if `bootstrap_seeds_applied?()` is true, prints “Seeds already applied (admin user exists). Skipping.” and returns without running bootstrap or dev seeds; otherwise unchanged behaviour.
  - Module docs updated for the new function and the skip behaviour.

- **`priv/repo/seeds.exs`**
  - Ensures the app is started (`Application.ensure_all_started(:mv)`).
  - If `Mv.Release.bootstrap_seeds_applied?()` is true, prints the same skip message and does not run bootstrap or dev seeds; otherwise runs as before (bootstrap + dev seeds in dev/test).
  - Comment at the top updated to describe the skip behaviour.

- **Documentation**
  - `CODE_GUIDELINES.md` §1.2.1: seeds run on every start but exit early when already applied; mentions `bootstrap_seeds_applied?/0`.
  - `docs/admin-bootstrap-and-oidc-role-sync.md`: run_seeds skips when admin user exists; description of `run_seeds/0` updated.
  - `CHANGELOG.md` [Unreleased]: new “Seeds run only when needed” entry under Changed.

## Definition of Done
### Code Quality
- [x] No new technical depths
- [x] Linting passed
- [x] Documentation is added where needed

### Accessibility
- [x] New elements are properly defined with html-tags *(no new UI)*
- [x] Colour contrast follows WCAG criteria *(no new UI)*
- [x] Aria labels are added when needed *(no new UI)*
- [x] Everything is accessible by keyboard *(no new UI)*
- [x] Tab-Order is comprehensible *(no new UI)*
- [x] All interactive elements have a visible focus *(no new UI)*

### Testing
- [x] Tests for new code are written *(existing seeds and release tests cover behaviour; idempotency test still passes when second run skips)*
- [x] All tests pass
- [x] axe-core dev tools show no critical or major issues *(no UI changes)*

## Additional Notes

- **Review focus:** Logic in `Mv.Release` and `priv/repo/seeds.exs`; the “already applied” check is a single DB read for the admin user. On failure (e.g. DB down), `bootstrap_seeds_applied?/0` returns `false`, so seeds run (safe for first deploy).
- **Suggested check:** Run `mix test test/seeds_test.exs test/mv/release_test.exs` to confirm seeds and release behaviour.

Reviewed-on: #475
Co-authored-by: Simon <s.thiessen@local-it.org>
Co-committed-by: Simon <s.thiessen@local-it.org>

CODE_GUIDELINES.md

@@ -284,13 +284,13 @@ end
 
 ### 1.2.1 Database Seeds
 
-Seeds are split into **bootstrap** and **dev**:
+Seeds are split into **bootstrap** and **dev**. They run on every start (e.g. `just run`, Docker entrypoint) but **exit early** if already applied so startup stays fast and no duplicate data is created.
 
-- **`priv/repo/seeds.exs`** – Entrypoint. Runs `seeds_bootstrap.exs` always; runs `seeds_dev.exs` only when `Mix.env()` is `:dev` or `:test`.
+- **`priv/repo/seeds.exs`** – Entrypoint. If the admin user (ADMIN_EMAIL or default) already exists, skips entirely (unless `FORCE_SEEDS=true`); otherwise runs `seeds_bootstrap.exs` and, in dev/test, `seeds_dev.exs`.
 - **`priv/repo/seeds_bootstrap.exs`** – Creates only data required for system startup: membership fee types, custom fields, roles, admin user, system user, global settings (including default membership fee type). No members, no groups. Used in all environments (dev, test, prod).
 - **`priv/repo/seeds_dev.exs`** – Creates 20 sample members, groups, and optional custom field values. Run only in dev and test.
 
-In production, running `mix run priv/repo/seeds.exs` executes only the bootstrap part (no dev seeds).
+In production, running `mix run priv/repo/seeds.exs` (or `Mv.Release.run_seeds/0`) executes only the bootstrap part when not yet applied (no dev seeds unless `RUN_DEV_SEEDS=true`). The “already applied” check uses `Mv.Release.bootstrap_seeds_applied?/0` (admin user exists). Set `FORCE_SEEDS=true` to re-run seeds even when already applied.
 
 ### 1.3 Domain-Driven Design