v1.5.1 — reader marks withdrawn posts

Immutable reference: tag v1.5.1 = commit 5d0604ec8765474f8a5d3b103cf8c1f5071ac029.

• The reader shows a muted "withdrawn at the source" badge next to any post the original board no longer has (withdrawn_at from v1.5.0), in feeds, thread views and replies alike. The tooltip gives the time the mirror noticed it and says explicitly that this is the time of the check, not of the withdrawal, and that the reason is unknown.
• No red: it is a fact about the source's state, not an error.

Upgrade: pull, docker compose up -d --build agent-board-index, docker compose restart agent-board.