fix: NaN telemetry writes, docker-compose port exposure, /v1/events docs

[SimmerV]

fix(storage): don't fail node writes on non-finite telemetry values

proto3's JSON mapping serializes non-finite floats as the strings
"NaN"/"Infinity"/"-Infinity"; asyncpg can bind neither those strings nor
real NaN/inf floats into numeric columns, so one bad sensor value made the
whole node + telemetry-current write fail. Observed in production: a node with
a faulty environment sensor lost all telemetry persistence, erroring daily.
Non-finite values are now mapped to NULL at the typed-column boundary in
_write_node_telemetry_current; everything else passes through unchanged.
Includes regression tests (TestFiniteOrNone).

fix(compose): stop publishing postgres/API/SPA ports on all interfaces

docker-compose.yml published postgres on 0.0.0.0:5432 with the committed
default password — and docker-published ports bypass host firewalls (ufw), so
any network-reachable host could read/write the DB. Nothing external needs it:
the app reaches postgres over the compose network, and the backup/migrate
scripts use docker exec.

• postgres: port mapping removed (comment documents the docker exec path and
  the 127.0.0.1 override option for host access)
• meshinfo (9000) and frontend (8000): bound to 127.0.0.1 — caddy is the
  public entry for both
• mosquitto: stays fully published by design (WAN mesh gateways publish
  uplinks to it), with a pointer to the broker-auth sample config

docs(api): document the /v1/events SSE endpoint

The SSE stream powering live map/chat/telemetry/coverage updates was absent
from API.md's endpoint tables.