Add v1 dashboard create endpoint via shared creation flow

[AnthonyMockler]

Is your feature request related to a problem? Please describe.
Yes. API v1 could list and retrieve dashboards, but could not create them, which blocked API-first workflows and forced UI-only creation.

Describe the solution you'd like
This PR adds POST /api/v1/dashboards/ in API v1 by reusing the existing dashboard creation flow rather than introducing a second create path.

Implemented behavior:

• Create support in v1 dashboard endpoint (django_project/geosight/data/api/v1/dashboard.py).
• Shared create service reused by both UI and API (django_project/geosight/data/services/dashboard_create.py, wired from django_project/frontend/views/admin/dashboard/create.py).
• Supports both form/json-style payload parsing and multipart (including optional icon upload).
• Permission behavior aligned with existing creator+ expectations; anonymous/non-creator creation denied.
• Transaction-safe create flow (rollback on relation-save failures).
• API docs/schema updated to reflect actual request/response contract for create.

Describe alternatives you've considered

• Keep creation UI-only: rejected because it does not enable API automation.
• Build separate API-only create flow: rejected to avoid drift from existing UI business logic.
• Redesign payload model first: rejected to keep change surface small and ship endpoint now.

Additional context
Design/implementation alignment used from our plan:

• Thin API layer; business logic in shared service.
• Reuse over rewrite.
• Contract/documentation updates in the same branch as behavior changes.
• Real integration verification on a running local GeoSight instance.

Scope delivered in this PR:

• Add create support to v1 dashboards endpoint.
• Reuse dashboard creation flow used by UI.
• Add targeted tests for success, permission denial, validation, multipart, and rollback.
• Update create endpoint docs/schema.

Out of scope (unchanged):

• Update/PATCH semantics.
• Full payload redesign.
• Unrelated dashboard permission/feature changes.
• Generic map viewer save-as-URL workflow.

Implemented acceptance checklist:

• POST /api/v1/dashboards/ is available in API v1.
• Endpoint requires authentication and creator+ role.
• Anonymous/public and non-creator requests are denied.
• JSON and multipart create flows both work.
• Failed relation save does not persist partial dashboard data.
• API docs reflect real request and response behavior.
• Tests cover happy path and failure cases.

Verification executed for this PR:

• docker exec geosight_branch_dev python manage.py test geosight.data.tests.api.v1.dashboard.DashboardPermissionTest --keepdb --noinput → OK (14 tests).
• cd playwright/ci-test && npx playwright test tests/creator/project_creation/api_v1_create.ts tests/contributor/project_creation/api_v1_create_denied.ts --project=creator-chromium --project=contributor-chromium → PASS (7) FAIL (0).
• Existing admin project creation suite was also run as a local regression signal and failed in this local seed baseline (tracked in issue comment), not in the targeted endpoint suites.

Closes #652