Index, probe, and announce LNURL mints in the API.

Wire discovery and probing for LNURL mints, add rate-limited POST /api/index
for user submissions, and optionally announce confirmed state to relays.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -156,6 +156,12 @@ FEDIMINT_OBSERVER_URL=https://observer.fedimint.org/api/federations
 # which is the literal BACKEND.md behaviour and ranks noticeably worse.
 SCORE_PRIOR_MEAN=3
 
+# How many mints one address may submit to `POST /api/index` in an hour. Ten is several
+# times what any honest use of the review dialog needs; raise it while exercising the
+# whole flow end to end, which trips ten in about a minute. Behind a proxy, the limiter
+# can only tell visitors apart if `X-Forwarded-For` is forwarded (see README, "API").
+# INDEX_RATE_LIMIT=10
+
 # ─── Tooling ─────────────────────────────────────────────────────────────────
 
 # Dev server Boneyard captures against (`pnpm bones`). Defaults to WEB_PORT.