[Phase 3] Wire DevContext.configure() to pyinfra deploy scripts

## Summary

Extract fixture data generation (users, access keys, scaling groups, resource presets) into a shared module. Currently duplicated between context.py and pyinfra fixture templates.

## Current Duplication

- `context.py` lines 429-455 (`load_fixtures()`), 940-959 (`configure_appproxy_fixture()`)
- `pyinfra deploy/cores/manager/deploy_fixtures.py` lines 81-126
- `pyinfra templates/fixtures/users.json.j2` lines 102-192
- Both generate identical JSON structures with same variable substitution:
- Superadmin/user UUIDs, emails, passwords, access/secret keys
- Default domain, resource policies, scaling groups
- wsproxy_addr pointing to appproxy coordinator

## Approach

Create `config_gen/fixtures.py` with:

- `build_user_fixtures(superadmin_params, user_params) -> dict`
- `build_scaling_group_fixture(appproxy_addr, appproxy_secret) -> dict`
- `build_resource_presets() -> dict`
- Returns dicts that can be serialized to JSON

## Acceptance Criteria

- Single source of truth for fixture data structure
- Both context.py and pyinfra use shared functions
- Default user credentials from shared_defaults

JIRA Issue: BA-5572

Compare context.py load_fixtures() and configure_appproxy_fixture() with pyinfra deploy/cores/manager/deploy_fixtures.py and pyinfra/templates/fixtures/users.json.j2. Start by tracing the duplicated fixture structures and shared_defaults usage, then verify that both context.py and pyinfra use the shared functions and produce the same JSON data with default credentials.

Written by the indexing model from the issue text.