docs: name unreadable database certificate files

[indykish]

Summary

• Document that unreadable PostgreSQL certificate files now identify the
  responsible database setting.
• Document that malformed inline certificate data is redacted from errors.

Verification

• make lint: documentation checks passed (22 tests OK).
• The remaining OpenAPI drift failure was reproduced unchanged on a clean
  origin/main worktree and is unrelated to this changelog-only diff.

Companion code PR: agentsfleet/agentsfleet#642

Greptile Summary

This PR adds an Aug 31 changelog entry describing improved PostgreSQL certificate diagnostics and redaction.

• Documents setting, parameter, and path details for unreadable certificate files.
• Documents redaction of malformed inline certificate data.

Confidence Score: 5/5

The PR appears safe to merge with no actionable issues identified.

The new changelog block is structurally complete, and the available repository context does not contradict its product-behavior claims or establish a required accompanying documentation change.

Important Files Changed

Filename	Overview
changelog.mdx	Adds a well-formed, self-contained release-note entry for database certificate error handling without introducing an actionable documentation defect.

_{Reviews (1): Last reviewed commit: "docs(changelog): name unreadable databas..." | Re-trigger Greptile}