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feat(bb_mdns): periodic browse refresh for stable peers (B1-87) (#178)

IDF's mdns_browse only invokes the notify callback on PTR state
changes (initial discovery, removal). Active advertisers re-announcing
with the same TTL are silently absorbed by IDF's cache, so consumers
never see "still alive" events for stable peers — knot's last_seen_us
stagnates and the UI shows good devices as stale.

Add a periodic browse-refresh timer that walks s_subs[], calls
mdns_browse_delete + mdns_browse_new for each in-use slot, and forces
a fresh PTR scan. Each responder produces a fresh on_peer; peers that
don't reply within the new query window fire on_removed before any
subsequent on_peer.

CONFIG_BB_MDNS_BROWSE_REFRESH_INTERVAL_S (default 60s, 0 disables)
controls cadence. Timer is started after lifecycle start in
bb_mdns_on_got_ip, and torn down alongside s_announce_timer in
bb_mdns_on_disconnect / bb_mdns_shutdown so it can't fire against
torn-down state. Refresh callback uses the same lock-snapshot-then-
release pattern as bb_mdns_re_arm_browses to avoid blocking the
dispatch task across IDF calls.

Header doc comment updated to call out the freshness contract:
on_peer fires on add + scheduled refresh, refresh churn (on_removed
followed by on_peer) is normal, finer freshness needs consumer-side
bb_mdns_query_txt. Smoke envs (esp32, esp32c3, tdongle, elecrow-p4-
hmi7) build clean.

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