Fix BASE_URL config loading, add tasks/projects; robust .env path resolution

- Config: try ENV_FILE, .env, ../.env for loading; trim trailing slash from BaseURL
- Log BASE_URL at server startup for verification
- .env.example: document BASE_URL
- Tasks, projects, tags, migrations and related API/handlers

Made-with: Cursor

migrations/000007_tasks.down.sql

@@ -0,0 +1,10 @@
+-- To-do list support rollback
+
+DROP TABLE IF EXISTS task_webhooks;
+DROP TABLE IF EXISTS task_reminders;
+DROP TABLE IF EXISTS task_dependencies;
+DROP TABLE IF EXISTS task_tags;
+DROP TABLE IF EXISTS tasks;
+DROP TABLE IF EXISTS tags;
+DROP TABLE IF EXISTS project_members;
+DROP TABLE IF EXISTS projects;