Expand ecash explorer capabilities

Add Fedimint discovery, dual SQLite/Postgres storage, richer review handling, and generated social imagery.

.env.example

@@ -39,15 +39,39 @@ PUBLIC_API_URL=
 SITE_URL=https://cashumints.space
 
 # ─── API storage ─────────────────────────────────────────────────────────────
-# Unset means api/data/, resolved from the source tree regardless of where you run
-# from. Set these and they are taken as given: a relative path resolves against the
-# working directory (api/ under every pnpm script), so prefer absolute paths. In
-# production point both at a directory the service user owns.
+# The API serves everything from its own database and nothing from a mint or a relay at
+# request time, so a restart loses nothing: mint metadata, cached /v1/info payloads,
+# reviews, probe history and the discovery cursor all live here. Icons live next to it
+# as files. In production point both at a directory the service user owns.
 
-# SQLite file.
+# Which database. Unset means SQLite at DB_PATH below, which is what every existing
+# deployment has and needs no setup at all. Set it to a postgres:// URL to use Postgres
+# instead — put the API on one host and the database on another, run more than one API
+# process, or fold the data into an existing backup and replication setup.
+#
+# Both backends create their tables on the first connection, so an empty database is
+# the whole installation. To carry existing data across, in either direction:
+#
+#   pnpm --filter ./api migrate --to postgres://user:pw@localhost:5432/cashumints
+#
+# then set DATABASE_URL to the same value and restart. Stop the API first: migrating
+# from a database that is still being written to copies a moving target.
+#
+# DATABASE_URL=postgres://cashumints:secret@localhost:5432/cashumints
+# DATABASE_URL=sqlite:/var/lib/cashumints/cashumints.db
+
+# SQLite file. Ignored when DATABASE_URL is set. Unset means api/data/, resolved from
+# the source tree regardless of where you run from. Set it and it is taken as given: a
+# relative path resolves against the working directory (api/ under every pnpm script),
+# so prefer an absolute path.
 # DB_PATH=/var/lib/cashumints/cashumints.db
 
-# Cached mint icons, served at /icons/*.
+# Postgres connections held open. Ignored by SQLite, which has exactly one. The default
+# of 10 is well above what one indexer plus the read endpoints need.
+# DB_POOL_MAX=10
+
+# Cached mint icons, served at /icons/*. Files, not rows — they are kept here whichever
+# database is in use, and `migrate` does not touch them.
 # ICON_DIR=/var/lib/cashumints/icons
 
 # ─── Nostr relays ────────────────────────────────────────────────────────────
@@ -99,9 +123,22 @@ DISCOVERY_INTERVAL_MIN=60
 # Mints probed in parallel.
 PROBE_CONCURRENCY=8
 
-# Per-mint request timeout, milliseconds.
+# Per-mint request timeout, milliseconds. Cashu mints only: a federation is not
+# fetched directly, see below.
 PROBE_TIMEOUT_MS=5000
 
+# ─── Fedimint ────────────────────────────────────────────────────────────────
+# A federation has no HTTP status endpoint of its own — confirming one is up means
+# being a Fedimint client — so its status is read from an outside checker instead,
+# once per probe cycle for every federation at once. Every row whose status came
+# from here records that fact and its page prints it, so nothing implies this site
+# opened a socket itself.
+#
+# Empty disables the lookup entirely: every federation then carries the `announced`
+# status, which is the honest one for "nothing checks this". It is never inferred
+# to be online or offline. See the "Ecosystems" section of README.md.
+FEDIMINT_OBSERVER_URL=https://observer.fedimint.org/api/federations
+
 # ─── Ranking ─────────────────────────────────────────────────────────────────
 
 # Bayesian prior mean C. The neutral 3 is intentional — read the "Ranking" section