feat: add federation gateway monitoring and API

[bansalayush247]

## Summary

This PR adds federation gateway monitoring, stores gateway announcements in the database, and exposes the stored gateway data through a new API endpoint.

## Changes

- add a new `GatewayInfo` API type for gateway responses
- add schema migration `v9` with a new `gateways` table
- start a background gateway monitor for each federation
- poll LN module peers for gateway announcements and merge results across guardians
- upsert gateway records with `first_seen` and `last_seen` timestamps
- expose `GET /federations/:federation_id/gateways`

## Steps to Run Locally

1. Enter the Nix shell.

```bash
cd /home/nightfury69/data/fedimint-observer
nix develop

2. Copy the env file once.

cp sample.env .env

3. Start PostgreSQL.

just pg_start

4. Export the .env values into your shell, then run the server.

set -a
source .env
set +a
cargo run --bin fmo_server

5. Register a live federation from another terminal inside nix develop.

set -a
source .env
set +a

INVITE=$(curl -s "https://observer.fedimint.org/api/federations" | python3 -c "
import sys, json
feds = json.load(sys.stdin)
print(feds[0]['invite'])
")

echo "Invite: $INVITE"

curl -X PUT http://localhost:3000/federations \
  -H "Authorization: Bearer $FO_ADMIN_AUTH" \
  -H "Content-Type: application/json" \
  -d "{\"invite\": \"$INVITE\"}"

6. Wait for the gateway monitor to poll.
   The default poll interval is 5 minutes, or it can be reduced temporarily for local testing.

7. Fetch the federation ID and test the gateways endpoint.

FED_ID=$(curl -s http://localhost:3000/federations | python3 -c "
import sys, json
feds = json.load(sys.stdin)
print(feds[0]['id'])
")

echo "Federation ID: $FED_ID"

curl -s http://localhost:3000/federations/$FED_ID/gateways | python3 -m json.tool

Verification

• manually registered a live federation locally
• confirmed the gateway monitor stores gateway data
• confirmed GET /federations/:federation_id/gateways returns the stored gateway list