feat: HTML landing page

[turban]

Closes #68.

What

`GET /` now returns an HTML landing page. `GET /?f=json` or a request with `Accept: application/json` returns the existing JSON `RootResponse` — the same content negotiation pattern used by TiTiler and OGC APIs.

Landing page content

• Instance status — configured extent (name, id, bbox, country code) or a setup prompt if none is configured
• Datasets table — all ingested datasets with variable, period type, temporal coverage, and publication status; a setup prompt if nothing has been ingested yet
• Navigation links
• Version badge in the header showing the current package version (`0.1.0a1`)

Implementation

• Template at `src/climate_api/templates/landing_page.html` — self-contained HTML/CSS, no external CDN dependencies, bundled as package data and loaded via `importlib.resources`
• `src/climate_api/system/templates.py` owns all root-resource representations: template loading/caching, content negotiation (`wants_json`), HTML rendering (`render_landing`), JSON navigation document (`root_json`), cached app version, and the OpenAPI `responses` schema constant. `routes.py` contains only the route handlers.
• Content negotiation respects RFC 7231 q-values: `Accept: application/json, text/html` → JSON; `Accept: text/html;q=1.0, application/json;q=0.9` → HTML
• Version resolved once at import time via `importlib.metadata.version("climate-api")` with a `PackageNotFoundError` fallback
• Exception handling in `render_landing` distinguishes expected config gaps (`ValueError` from extent) from unexpected errors (logged via `_log.exception`)
• All template links use `{{ base }}` prefix so the page works correctly behind a reverse proxy subpath
• `jinja2` added as an explicit dependency (was already a transitive dep via Starlette)