fix(cluster): stamp liveness on the database clock and refuse undialable addresses

DiscoverAdvertisedAddr promised to return an error rather than a fallback
no peer can dial, but only rejected an unspecified address. With PostgreSQL
on the same host or pod as a replica, which is compose, single-node and any
sidecar layout, the route to it is loopback, so every replica advertised
127.0.0.1 and a peer dialling that reached itself. Loopback, link-local and
zoned source addresses are now rejected with an error naming the remedy, and
a port outside 1-65535 is rejected before it becomes an undialable address.

Liveness was also measured on each replica's own clock: Register and
Heartbeat stamped last_seen from the Go process, and Live compared those
rows against the reading replica's time.Now(). Skew therefore shrank or
stretched the window by writerBehind+readerAhead, evicting healthy peers or
keeping dead ones. Both sides now use the database clock, which is the one
clock every replica demonstrably shares.

Assisted-by: Claude Opus 5 [claude-code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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