Store user/password in encrypted Node-RED credentials

[ukmoose]

Moves the PostgreSQL user and password from plain-text storage in flows.json to Node-RED's encrypted credential system (flows_cred.json).
What changed:

• User and password are now stored as Node-RED credentials (encrypted at rest)
• A dropdown selector in the Security tab lets users choose between credential, env variable, or global context for each field
• Existing flows are automatically migrated on first load — plain-text values are copied to credentials and a warning is logged prompting the user to deploy
• On deploy, the old plain-text values are removed from flows.json automatically
• Credentials take priority over flows.json values
• Config node label still shows the username
• Added documentation to README and in-editor help for the config node

Breaking change: version bumped to 0.16.0 as the config storage format has changed.
Closes #137

Tested by taking flows.json stored username & password and env stored username & password, upgrading to patched node,
ensuring that both still can access db, and checking flows.json for username & password values.

[Alkarex]

Thanks! Looking into it.
Related:

• Store PostgreSQL user and password in Node-RED credentials #136

[Alkarex]

I have made some changes but not tested thoroughly.
Could you please give it a try again?