feat(marketing): add "every one named, licence-reviewed and linkable" beside the source count

## What to add
One line on the landing page, next to the source count:

> **534 sources — every one named, licence-reviewed and linkable.**

## Why one line is worth an issue
It is the single highest-leverage sentence available, because it answers the
exact objection a prospect raised about the market leader **before they have to
ask it**. Dataminr cannot write this sentence: their model depends on
privileged, contractually restricted source access, which is precisely why they
cannot show provenance.

Pair it with the count reconciliation (534 vs 536 vs "500+") so the number is
consistent everywhere it appears.

## Acceptance
- [ ] Line live on the landing page
- [ ] Links to `/sources`
- [ ] Count matches every other surface

Effort: XS (~0.25d).

Start at the landing page implementation and compare its source count with the /sources page and other count surfaces. Done means the sentence appears beside the count, links to /sources, and the displayed count is consistent everywhere.

Written by the indexing model from the issue text.