Add Nostr open-discovery startup sweep with diagnostic logging

Under `node.discovery.nostr.policy: open`, the per-tick auto-dial in
`queue_open_discovery_retries` was supposed to pick up adverts cached
from the relay subscription backlog at startup, but in practice only
adverts arriving live (after the daemon was up) were being dialed.
Backlog adverts sat in the in-memory cache until they aged out.

Adds a one-shot startup sweep that runs once per daemon start, gated
identically to the per-tick sweep (`enabled` && `policy == open`),
after a configurable settle delay so the relay subscription backlog
has time to populate the advert cache. The sweep iterates the cache
with the same skip-filters as the per-tick path (statically-configured
peers, already-connected, retry-pending, connecting) plus a tighter
age filter: only adverts whose `created_at` is within
`startup_sweep_max_age_secs` of now are queued.

Two new config fields under `node.discovery.nostr`:
- `startup_sweep_delay_secs` (default 5)
- `startup_sweep_max_age_secs` (default 3600 = one hour)

Both are only consulted when `policy == open`; under any other
policy the sweep is a no-op.

Adds diagnostic logging to the open-discovery sweep so operators can
verify what the auto-dial path is doing on each daemon bring-up:
info-level on each retry-queued enqueue (with peer short-npub and
advert age), and a one-line summary on every startup sweep and on
any per-tick sweep that queues at least one retry. The summary
breaks down skipped candidates by reason (age, configured, self,
already-connected, retry-pending, connecting, no-endpoints,
invalid-npub) — currently the path was silent so there was no
operator-visible signal that the cache iteration was running.

Refactors the existing `queue_open_discovery_retries` body into a
shared `run_open_discovery_sweep(max_age_secs, caller)` helper so
the per-tick and startup paths share filter/queue logic and only
differ in the age filter and log label. Surfaces `created_at` from
`NostrDiscovery::cached_open_discovery_candidates` (return tuple
extended) so the age filter has the data it needs.

Three new unit tests in `config::node::tests` cover the new defaults,
YAML override round-trip, and partial-YAML default fallback.

docs/design/fips-configuration.md

@@ -205,6 +205,8 @@ without that feature ignore `udp:nat` bootstrap configuration.
 | `node.discovery.nostr.punch_duration_ms` | u64 | `10000` | How long to keep punching before failure |
 | `node.discovery.nostr.advert_ttl_secs` | u64 | `3600` | Advert TTL in seconds |
 | `node.discovery.nostr.advert_refresh_secs` | u64 | `1800` | How often adverts are refreshed in seconds |
+| `node.discovery.nostr.startup_sweep_delay_secs` | u64 | `5` | Settle delay after Nostr discovery starts before the one-shot startup advert sweep runs (only used under `policy: open`). Allows the relay subscription backlog to populate the in-memory advert cache before the sweep fires |
+| `node.discovery.nostr.startup_sweep_max_age_secs` | u64 | `3600` | Maximum advert age (`now - created_at`) considered by the one-shot startup sweep (only used under `policy: open`). Adverts older than this are skipped on startup; the per-tick sweep still considers them up to `valid_until_ms` |
 
 If `stun_servers` is omitted, the built-in default list above is used. If it is
 specified in YAML, the configured list fully overrides the defaults.