Document SQLite database storage and DB_PATH - #520

## Release `3.1.0`

Prepares the version numbers and release notes for `3.1.0`.

**Changelog draft:**

---

**SQLite Database Integration 3.1 is here! 🎉**

This release improves **database storage and configuration** and fixes
several MySQL compatibility issues.

## What's new

Version 3.1 improves how WordPress sites store, locate, and protect
their SQLite databases. It also fixes SQL behavior and export
compatibility:

- **Randomized database paths:** The default database location is now a
randomized directory under `wp-content/database/`, recorded in
`wp-content/database/db-path.php`.
([#502](#502))
- **`DB_PATH`:** Configure the database with one full-path constant,
also available at runtime for integrations.
([#512](#512))
- **SQL compatibility:** Fix `IF()` condition evaluation and make
`TRADITIONAL` enable its component SQL modes.
([#518](#518),
[#509](#509))
- **WordPress table collations:** Default to `utf8mb4_unicode_520_ci`
for new tables created with WordPress's charset settings, improving
exports to MariaDB. Existing tables keep their recorded collation.
([#514](#514))
- **Documentation:** A new plugin README and expanded FAQ explain
database storage and secure configuration.
([#520](#520))

For more information about database paths and secure configuration, read
the [database storage
guide](https://github.com/WordPress/sqlite-database-integration/blob/trunk/packages/plugin-sqlite-database-integration/README.md#database-storage).

## Upgrading to 3.1

Upgrading an existing SQLite site is straightforward:

1. **Back up** your SQLite database.
2. **Update the plugin** to version 3.1.

Existing `.ht.sqlite` and `.ht.sqlite.php` databases move to the
randomized layout automatically unless a database file path is
explicitly configured.

To properly **secure the database**, set `DB_PATH` in `wp-config.php` to
an absolute file path outside the web root that your web server does not
expose. Its directory must be writable by PHP. Changing `DB_PATH` does
not move an existing database.

## Breaking changes

Review these changes if you use custom database settings or
integrations:

- **Database paths:** Default database files now move to randomized
paths under `wp-content/database/`. Explicitly configured file paths
stay unchanged. Integrations, including backup and migration tools, must
read `DB_PATH` after WordPress loads instead of assuming a fixed
filename.
- **Legacy constants:** `DB_DIR` and `DB_FILE` are now deprecated. They
and the previously deprecated `FQDB` and `FQDBDIR` remain supported, but
`DB_PATH` takes precedence. Conflicting values trigger warnings.
- **Absolute paths:** Relative database file and directory paths are now
rejected. `:memory:` remains available for in-memory databases.

## Thank you

Thank you to everyone who contributed, tested, and helped update
integrations.

**Changes since 3.0.2:**
[`v3.0.2...v3.1.0`](v3.0.2...v3.1.0)

---

**PR comparison:**
[`v3.0.2...release/v3.1.0`](v3.0.2...release/v3.1.0)

## Next steps

1. **Review** the release changes and changelog draft.
2. **Merge** this pull request to complete the release.

Merging will automatically build the plugin ZIP, create a [GitHub
release](https://github.com/WordPress/sqlite-database-integration/releases),
and deploy to
[WordPress.org](https://wordpress.org/plugins/sqlite-database-integration/).