Add randomized SQLite database storage

[JanJakes]

Summary

This PR adds randomized storage for SQLite databases as an additional layer of protection against direct web access.

• Randomized paths: New managed databases are stored in a directory generated from 128 bits of randomness.
• Portable discovery: db-path.php records the database location relative to the managed database root.
• Automatic migration: Existing .ht.sqlite and .ht.sqlite.php databases are moved to the randomized layout.
• Explicit path compatibility: User-configured database paths and :memory: continue to work unchanged.
• Safer failures: Filesystem errors do not expose the randomized database path.

Managed storage

When no explicit database path is configured, the default layout is:

wp-content/
└── database/
    ├── .htaccess
    ├── index.php
    ├── .ht.sqlite.lock
    ├── db-path.php
    └── .ht.<32-hex-char-random-key>/
        ├── .htaccess
        ├── index.php
        └── .ht.sqlite

The db-path.php file returns the database location using __DIR__, so copying or moving the complete database directory keeps the reference valid. The storage manager also restores missing directories, protection files, and the database file when needed.

Legacy migration

Migration is serialized across concurrent requests. Before moving a legacy database, the storage manager checkpoints WAL data, switches to DELETE journal mode, and acquires an exclusive SQLite lock. If the database remains busy or migration otherwise fails, the original database file stays in place.

Why

A SQLite database at a predictable location under the document root may be served directly when the web server does not honor .htaccess or equivalent denial rules. The randomized directory makes accidental exposure substantially harder while retaining a stable discovery mechanism for WordPress and external tools.

This is an additional safeguard, not a replacement for private storage. Keeping the database outside the document root or configuring the web server to deny access remains the strongest protection.

[adamziel]

The idea looks great. I don't have a good understanding why is the .ht.sqlite.lock file outside of the .ht.<hash> directory, but that also doesn't matter too much. I won't be able to review this deeply before my afk.