fix: don't log admin password via slog (closes #26)

Replace slog.Info (which outputs structured JSON in prod and ends up in
log aggregation) with a plain fmt.Fprintf to stderr. The password is
printed once on first startup in a clearly-delimited banner that won't
be parsed as a structured log field.

internal/database/database.go

@@ -3,7 +3,9 @@ package database
 import (
 	"context"
 	"database/sql"
+	"fmt"
 	"log/slog"
+	"os"
 
 	"go.uber.org/fx"
 	"gorm.io/driver/sqlite"
@@ -118,11 +120,18 @@ func (d *Database) migrate() error {
 			return err
 		}
 
-		// Log the password - this will only happen once on first startup
-		d.log.Info("admin user created",
-			"username", "admin",
-			"password", password,
-			"message", "SAVE THIS PASSWORD - it will not be shown again!")
+		// Print the password directly to stderr so it never ends up in
+		// structured JSON log aggregation. This message is only shown
+		// once on first startup.
+		fmt.Fprintf(os.Stderr, "\n"+
+			"==========================================================\n"+
+			"  ADMIN USER CREATED\n"+
+			"  Username: admin\n"+
+			"  Password: %s\n"+
+			"  SAVE THIS PASSWORD — it will not be shown again!\n"+
+			"==========================================================\n\n",
+			password,
+		)
 	}
 
 	return nil