Fix #149: honor THINGSDB before database discovery

[fazmulla66-bot]

Summary

• prefer the explicit THINGSDB environment override before automatic Things App Group discovery
• preserve existing glob-based discovery when no override is configured
• add regression coverage for both paths

This prevents import things from stalling in filesystem discovery when callers have already supplied an exact database path.

Fixes #149

Test plan

• pytest -q — 36 passed
• documentation/doctest suite — 57 passed, 1 skipped
• pycodestyle things/*.py tests/*.py
• pydocstyle things/*.py tests/*.py
• integration-tested through a local Things MCP against a live read-only Things database

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud