Sources: per-source metrics in each card; drop the aggregated Home/Maintenance blocks

[jozef2svrcek]

Summary

Show each source's update metrics inside its own card (Ajedrez, TWIC, Lichess Broadcasts) on the Sources page, instead of in a separate aggregated block elsewhere.

Desired behaviour

• Each source card shows: latest item + date (TWIC issue / Lichess month / Ajedrez part), games imported, and item count.
• Remove both aggregations added in Home: "Latest TWIC issue" tile is TWIC-only — generalise for multi-source updates #176/Home: per-source "Latest updates" list instead of TWIC-only tile (#176) #193 (decided): the Home "Latest updates" list and the Maintenance "By source" block.
• Keep the backing backend fields from Home: "Latest TWIC issue" tile is TWIC-only — generalise for multi-source updates #176/Home: per-source "Latest updates" list instead of TWIC-only tile (#176) #193 (SourceStatus.last_import, imported_games) — the cards consume them.

Notes

• Supersedes the UI parts of Home: "Latest TWIC issue" tile is TWIC-only — generalise for multi-source updates #176/Home: per-source "Latest updates" list instead of TWIC-only tile (#176) #193; those PRs' backend fields remain the foundation. If Home: per-source "Latest updates" list instead of TWIC-only tile (#176) #193 hasn't merged yet, this can be folded in rather than adding-then-removing the aggregations.

Related

#176, #193 (last_import / imported_games backend + the aggregations this replaces).