Adds a last_read_at watermark to the sessions table so surfaces (CLI,
TUI, desktop) can badge unread conversations. Read state derives from
the watermark vs latest activity, so new messages flip a conversation
back to unread with zero writes on the message path. NULL means never
tracked, so shipping the column doesn't badge pre-existing history.
set_session_read() stamps the whole compression lineage, matching the
archive/pin semantics; list_sessions_rich() rows carry a derived
`unread` key. DB layer only — no surface exposes it yet.
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