add postgresql and mysql integrations

[hkonsti]

Adds PostgreSQL and MySQL (MariaDB too) as two integrations on one code path. A database is connected with its host, port, database, user, password and encryption, signing in first with a Test connection like the mailbox form; agents get its tables as a tree, a table holding its rows, each written as "column: value" lines and found again by its primary key, and a query tool that runs one statement that reads inside a read-only transaction, 200 rows back as JSON lines.

The host is resolved and checked before connecting, so a connection can't be pointed at something on the api's own network: private addresses are refused unless the new ALLOW_PRIVATE_HOSTS variable says otherwise, and the connection goes to the checked address with the typed host used for certificate verification. MySQL's read-only transaction lets DDL through, so the statement check (one statement, starting with SELECT, WITH, SHOW, EXPLAIN, DESCRIBE, TABLE or VALUES) is what keeps a query to reading; the docs ask for a read-only database user as well.

Along the way: a collection's read action became optional and a new command can be marked readOnly, for the queries collection; the api's error envelope now carries message where an error has one, so agents see the database's own words; Bun's SQL client is given an explicit URL, since with only host and port fields it merges the api's own DATABASE_URL query parameters into the connection. Both databases were tested end to end against local Postgres 17 and MariaDB 10.11 servers with TLS, and the pages checked in a browser.

---

Open workspace in Conductor