fix(nostr): exponential reconnect backoff so unreachable relays stop exhausting the thread pool (#191) - #201

Fix #2 for #191. Builds on the reconnect rework already merged to main.

What #191 asked for

The Nostr relay service retried unreachable relays with no backoff. Each attempt resolves DNS and spawns a _thread, and the ESP32-S3 pool is tiny, so hammering a dead relay starved other apps' networking (this is what broke audio playback in The Free Lantern Player). The DNS cache (#189) was fix #1; this is fix #2.

What was still wrong on main

main already normalises reconnect_interval and gates on _network_available(), which handles the "Wi-Fi fully down" case. But two gaps remained when Wi-Fi is up and a specific relay is unreachable or flapping:

1. Clean-close tight loop. When a relay accepts the socket then drops it, _connect_and_run() returns without raising, so the except branch (with its sleep) never runs. The while self.running: loop reconnects with zero delay and hammers.
2. No backoff. The exception path waits a fixed reconnect_interval (3s) forever, so a permanently dead relay keeps retrying every 3s.

The fix

Exponential backoff in _async_main, seeded from reconnect_interval:

• Wait before every reconnect (not just after an exception), which closes the clean-close zero-delay loop.
• connected_ok tracks whether the just-ended session was actually live: reset the delay to the seed interval after a live close, otherwise double it toward a 300s ceiling. An unreachable relay backs off (3s -> 6 -> 12 -> ... -> 300) instead of spinning; a healthy relay that blips recovers fast.
• No relay is abandoned, so one that recovers reconnects on its own.

The backoff math is a pure helper (_next_backoff) so it is unit-testable without a network. reconnect_interval <= 0 still disables reconnect, and _network_available() gating is unchanged.

Scope is exactly the Nostr relay service: WebSocketApp is only used by nostr/relay.py (nostr app + NWC both go through it).

Testing

• New tests/test_uaiowebsocket_backoff.py: 5/5 pass on the desktop build (./tests/unittest.sh tests/test_uaiowebsocket_backoff.py), network-free. Covers geometric growth, the 300s cap, reset-on-live-connection, and a custom seed interval as the floor.
• mpy-cross compiles clean; ASCII-only, MicroPython-compatible.