[kaicen-x]

The non-SQLite pool was hard-coded at 100 connections, so operators of
busy instances could not raise it without patching the source, and
small instances had no way to lower it.

• pkg/conf: add Database.MaxOpenConns, settable via
  CR_CONF_Database.MaxOpenConns or config.yaml
• inventory: size the pool from that value for non-SQLite databases
  and fall back to 100 when it is unset or out of the sane range
  (<= 10 or > 5000); SQLite keeps its 10-connection cap
• docker-compose: ship 200 as the stack default and 50 for dev
• drop trailing whitespace on the postgres image line

[Dvorinka]

Superseded by #409.

The pool is now configurable via Database.MaxOpenConns / CR_CONF_Database.MaxOpenConns. Unset or 0 keeps the old default of 100, values from 1 through 5000 are honored, and anything higher is clamped with a warning. SQLite stays on its own 10-connection cap.

This patch rejected every value <= 10, so a small instance still could not choose a pool under 11, and an out-of-range value silently fell back to 100 with no log. #409 covers the same docker-compose defaults (200 for the stack, 50 for dev) and adds a unit test for the sizing rules.