fix: verify the metadata database certificate with sslmode=verify-full

- Switch every `metadata_db_sslmode` passed to bincraft in `.crow/` and `local/` from `require` to `verify-full`.
- Pass `sslrootcert = bincraft::postgres_sslrootcert("verify-full")` alongside `verify-full` in the scripts that call `DBI::dbConnect()` themselves, since libpq does not read the system trust store.
- Note in the README that the endpoint serves a Let's Encrypt certificate and how to verify it.

Part of rpkgs/bincraft#225.

local/list-unsafe-flat-objects.R

@@ -30,7 +30,8 @@ con <- DBI::dbConnect(
   port = 15432L,
   user = "rpkgs",
   password = Sys.getenv("PGPASS"),
-  sslmode = "require"
+  sslmode = "verify-full",
+  sslrootcert = bincraft::postgres_sslrootcert("verify-full")
 )
 on.exit(try(DBI::dbDisconnect(con), silent = TRUE), add = TRUE)