Add source landing pages and feeds

[nuest]

Allows to explore and follow a specific source.

This is where #164 can then be shown.

[nuest]

Transferred from #164: the source landing page should display coverage statistics from SourceCoverageSnapshot. Suggested content for the statistics section:

Per-source coverage panel (data already computed weekly and available via GET /api/v1/sources/<id>/ → latest_coverage):

Field	Description
OPTIMAP works	Number of published works harvested from this source
OpenAlex total	Total works indexed in OpenAlex for this source
Coverage	optimap_count / openalex_total as a percentage
With geometry	% of OPTIMAP works that have spatial extent
With temporal extent	% of OPTIMAP works that have a time period
Open access	% of OPTIMAP works with gold/green/hybrid OA status
Contributors	Number of distinct users who contributed metadata for this source's works
Last updated	computed_at from the snapshot

A small time-series sparkline showing coverage % over time would also be useful (the weekly snapshots are stored in SourceCoverageSnapshot and retrievable via the API).

Note: coverage stats are only meaningful for sources with an OpenAlex ID (openalex_id set). Sources without one will show openalex_total: null.

[nuest]

Also add links from the statistics page to these pages.