nix,spindle: use `DATA_DIR` for db & other spindle data

This single persistent directory can be used for storing general spindle
data like db, motd file and upcoming sparse-clone git repos.

db path will be `${DATA_DIR}/spindle.db`

Signed-off-by: Seongmin Lee <[e-mail]>

nix/modules/spindle.nix

···
             description = "Address to listen on";
           };
 
-          dbPath = mkOption {
+          stateDir = mkOption {
             type = types.path;
-            default = "/var/lib/spindle/spindle.db";
-            description = "Path to the database file";
+            default = "/var/lib/spindle";
+            description = "Tangled spindle data directory";
           };
 
           hostname = mkOption {
···
           StateDirectory = "spindle";
           Environment = [
             "SPINDLE_SERVER_LISTEN_ADDR=${cfg.server.listenAddr}"
-            "SPINDLE_SERVER_DB_PATH=${cfg.server.dbPath}"
+            "SPINDLE_SERVER_DATA_DIR=${cfg.server.stateDir}"
             "SPINDLE_SERVER_HOSTNAME=${cfg.server.hostname}"
             "SPINDLE_SERVER_PLC_URL=${cfg.server.plcUrl}"
             "SPINDLE_SERVER_DEV=${lib.boolToString cfg.server.dev}"

nix/vm.nix

···
           };
         in {
           knot = mkDataSyncScripts "/mnt/knot-data" config.services.tangled.knot.stateDir;
-          spindle = mkDataSyncScripts "/mnt/spindle-data" (builtins.dirOf config.services.tangled.spindle.server.dbPath);
+          spindle = mkDataSyncScripts "/mnt/spindle-data" config.services.tangled.spindle.server.stateDir;
         };
       })
     ];

spindle/config/config.go

···
 import (
 	"context"
 	"fmt"
+	"path/filepath"
 
 	"github.com/bluesky-social/indigo/atproto/syntax"
 	"github.com/sethvargo/go-envconfig"
···
 
 type Server struct {
 	ListenAddr  string     `env:"LISTEN_ADDR, default=0.0.0.0:6555"`
-	DBPath      string     `env:"DB_PATH, default=spindle.db"`
 	Hostname    string     `env:"HOSTNAME, required"`
 	TapUrl      string     `env:"TAP_URL, required"`
 	PlcUrl      string     `env:"PLC_URL, default=https://plc.directory"`
···
 	Owner       syntax.DID `env:"OWNER, required"`
 	Secrets     Secrets    `env:",prefix=SECRETS_"`
 	LogDir      string     `env:"LOG_DIR, default=/var/log/spindle"`
+	DataDir     string     `env:"DATA_DIR, default=/var/lib/spindle"`
 	QueueSize   int        `env:"QUEUE_SIZE, default=100"`
 	MaxJobCount int        `env:"MAX_JOB_COUNT, default=2"` // max number of jobs that run at a time
 }
 
 func (s Server) Did() syntax.DID {
 	return syntax.DID(fmt.Sprintf("did:web:%s", s.Hostname))
+}
+
+func (s Server) DBPath() string {
+	return filepath.Join(s.DataDir, "spindle.db")
 }
 
 type Secrets struct {