chore: upgrade to 3.1.0+v2.0.0

Minor bump — no operator action required (Postgres/ClickHouse changes are automatic).

- Postgres: use pgautoupgrade/pgautoupgrade:18-alpine in place of the custom
  pg_upgrade entrypoint. The existing cluster is upgraded in place automatically
  on deploy; PGDATA pinned to the legacy path; adds a pg_isready healthcheck.
  Removes entrypoint.postgres.sh.tmpl and DB_ENTRYPOINT_VERSION.
- ClickHouse backup fetch: cache the clickhouse-backup binary on the persistent
  volume and retry with backoff to avoid the download crash-loop. The tool is
  required — if it can't be installed after retries the entrypoint aborts and
  the server does not start, rather than coming up without backup/restore.
- Add CLICKHOUSE_DATABASE_URL; bump the clickhouse entrypoint config version.
- Remove a stray broken link reference in the README.

entrypoint.clickhouse.sh

@@ -1,6 +1,14 @@
 #!/bin/bash
 
-set -ex
+# clickhouse-backup is a backup tool (backupbot pre/post-hooks: `clickhouse-backup create/restore`).
+# It is a 22 MB GitHub download (rate-limit / network), which can fail to download, and lead to crash loop and download throttling. 
+#
+# to make the download smoother:
+#   - cache the binary on the persistent clickhouse data volume (/var/lib/clickhouse) so it is fetched
+#     at most once and reused on every container restart (no re-download amplification);
+#   - retry with backoff to ride out transient GitHub failures
+
+set -e
 
 CLICKHOUSE_BACKUP_VERSION=2.4.2
 
@@ -17,13 +25,33 @@ elif [[ $ARCH =~ "x86_64" ]]; then
   ARCH="amd64"
 fi
 
-wget \
-    --quiet \
-    --continue \
-    --no-clobber \
-    --output-document=/tmp/clickhouse-backup.tar.gz \
-    "https://github.com/AlexAkulov/clickhouse-backup/releases/download/v${CLICKHOUSE_BACKUP_VERSION}/clickhouse-backup-linux-${ARCH}.tar.gz" 2>/dev/null
+CACHE_DIR=/var/lib/clickhouse/.ccci-bin
+CACHED="${CACHE_DIR}/clickhouse-backup"
+BIN=/usr/local/bin/clickhouse-backup
+URL="https://github.com/AlexAkulov/clickhouse-backup/releases/download/v${CLICKHOUSE_BACKUP_VERSION}/clickhouse-backup-linux-${ARCH}.tar.gz"
 
-tar -xf /tmp/clickhouse-backup.tar.gz --directory=/usr/local/bin --strip-components=3
+install_clickhouse_backup() {
+  mkdir -p "$CACHE_DIR"
+  if [ -x "$CACHED" ]; then
+    cp -f "$CACHED" "$BIN"
+    echo "clickhouse-backup: restored from persistent cache ($CACHED)"
+    return 0
+  fi
+  for attempt in 1 2 3 4 5; do
+    if wget --continue --output-document=/tmp/clickhouse-backup.tar.gz "$URL" \
+       && tar -xf /tmp/clickhouse-backup.tar.gz --directory=/usr/local/bin --strip-components=3; then
+      cp -f "$BIN" "$CACHED" 2>/dev/null || true
+      echo "clickhouse-backup: downloaded + cached (attempt ${attempt})"
+      return 0
+    fi
+    echo "clickhouse-backup: fetch attempt ${attempt} failed; backing off $((attempt * 10))s" >&2
+    sleep $((attempt * 10))
+  done
+  echo "clickhouse-backup: fetch FAILED after all retries — aborting; clickhouse-server will NOT start (backup tool is required)" >&2
+  return 1
+}
 
-/entrypoint.sh
+#if the backup tool cannot be installed after retries, it aborts (set -e) so the deploy fails
+install_clickhouse_backup
+
+exec /entrypoint.sh