docs: add Ubuntu sqlite3 installation and bind-mount volume custom guides to README.md

- Document host bind-mount configuration (./volumes/data:/app/data) in README.md §5.5
- Add Ubuntu / Linux prerequisite tools installation guide (sudo apt install sqlite3 curl make) in README.md §6
- Document zero-install SQLite dump restore command via docker compose exec in README.md §7.3
- Add /volumes/ and database ignores to root .gitignore

.gitignore

@@ -7,6 +7,13 @@
 
 # Binary zip archives
 *.zip
+
+# Local SQLite databases and bind-mount volumes
+/volumes/
+*.db
+*.db-wal
+*.db-shm
+!backend/db/backup/*.sql
 # >>> MAM managed block (managed by install.sh — do not edit) >>>
 /.venv/
 /.mam/