Test and document max number of active runs and instances supported by the server

[r4victor]

Users should know how many instances and runs the dstack server can process simultaneously. For this, the dstack server should be tested under an active load in both SQLite and Postgres setups. The estimation can be conservative, i.e. we should provide numbers that the server is guaranteed to handle without degraded performance/errors.

In addition, the documentation should include the minimum/recommended server requirements (CPU/RAM).