fix: unify DATA_DIR default to /var/lib/webhooker for all environments

Remove devDataDir() XDG-based logic. Both dev and prod now default
DATA_DIR to /var/lib/webhooker. Update Dockerfile and README to match.

internal/config/config.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"log/slog"
 	"os"
-	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -80,23 +79,6 @@ func envInt(key string, defaultValue int) int {
 	return defaultValue
 }
 
-// devDataDir returns the default data directory for the dev
-// environment. It uses $XDG_DATA_HOME/webhooker if set, otherwise
-// falls back to $HOME/.local/share/webhooker. The result is always
-// an absolute path so the application's behavior does not depend on
-// the working directory.
-func devDataDir() string {
-	if xdg := os.Getenv("XDG_DATA_HOME"); xdg != "" {
-		return filepath.Join(xdg, "webhooker")
-	}
-	home, err := os.UserHomeDir()
-	if err != nil {
-		// Last resort: use /tmp so we still have an absolute path.
-		return filepath.Join(os.TempDir(), "webhooker")
-	}
-	return filepath.Join(home, ".local", "share", "webhooker")
-}
-
 // nolint:revive // lc parameter is required by fx even if unused
 func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 	log := params.Logger.Get()
@@ -127,16 +109,11 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		params:          &params,
 	}
 
-	// Set default DataDir based on environment. All SQLite databases
-	// (main application DB and per-webhook event DBs) live here.
-	// Both defaults are absolute paths to avoid dependence on the
-	// working directory.
+	// Set default DataDir. All SQLite databases (main application DB
+	// and per-webhook event DBs) live here. The same default is used
+	// regardless of environment; override with DATA_DIR if needed.
 	if s.DataDir == "" {
-		if s.IsProd() {
-			s.DataDir = "/data"
-		} else {
-			s.DataDir = devDataDir()
-		}
+		s.DataDir = "/var/lib/webhooker"
 	}
 
 	if s.Debug {