add postgresql backup hooks

clanModules/postgresql/README.md

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+A free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance.
+---

clanModules/postgresql/default.nix

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+{
+  pkgs,
+  lib,
+  config,
+  ...
+}:
+let
+  createDatatbaseState =
+    db:
+    let
+      folder = "/var/backup/postgresql/${db}";
+      curFile = "${folder}/dump.sql.zstd";
+      prevFile = "${folder}/dump.sql.prev.zstd";
+      inProgressFile = "${folder}/dump.sql.in-progress.zstd";
+    in
+    {
+      folders = [ folder ];
+      preBackupCommand = ''
+        (
+          umask 0077 # ensure backup is only readable by postgres user
+          if [ -e ${curFile} ]; then
+            mv ${curFile} ${prevFile}
+          fi
+          pg_dump -C ${db} | \
+            ${pkgs.zstd}/bin/zstd --rsyncable | \
+            > ${inProgressFile}
+          mv ${inProgressFile} ${curFile}
+        )
+      '';
+      postRestoreCommand = ''
+        if [[ -f ${prevFile} ]]; then
+          zstd --decompress --stdout ${prevFile} | psql -d ${db}
+        fi
+      '';
+    };
+in
+{
+  options.clan.postgresl = {
+    databases = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ "clan" ];
+    };
+  };
+  config = {
+    services.postgresql.enable = true;
+    clanCore.state = lib.listToAttrs (
+      builtins.map (
+        db: lib.nameValuePair "postgresql-${db}" (createDatatbaseState db)
+      ) config.clan.postgresl.databases
+    );
+  };
+}