docs: add PostgreSQL troubleshooting queries

[monodera]

Summary

• Add a new PostgreSQL Troubleshooting section to docs/examples/queries.md
• Covers: running process inspection, lock detection, process termination, table/index size checks, and safe DDL operations on large tables
• All queries reviewed and corrected (fixed idle-state filtering, state_change vs query_start, explicit COUNT alias)

Test plan

• Verify the docs render correctly (mkdocs serve)
• Spot-check SQL syntax for each query

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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