docs: add common MongoDB query reference (closes #76)

[Mrsandeep27]

Summary

Closes #76. Adds docs/common-db-queries.md — a quick-reference playbook of the MongoDB queries Mantis users most frequently need, grouped by collection.

Directly covers the three examples called out in the issue:

• Extract all TLDs under an org — List every TLD under an organisation
• List all subdomains for a given TLD — All subdomains for a given TLD
• High and Critical security issues for an org — High and Critical severity issues for an organisation

…and adds ~20 more queries that tend to come up in practice.

What's in the doc

Assets

• List every TLD under an org
• TLD count grouped by org
• All subdomains for a given TLD (with regex-escape note)
• Subdomain count per TLD (aggregation)
• Assets behind a CDN (Cloudflare etc.)
• Assets protected by a WAF
• Assets exposing specific ports (SSH, MySQL, Redis example)
• Assets by ASN / hosting provider (AWS example)
• Stale (decommissioned) assets
• Recently discovered assets (last 7 days)
• Technology usage breakdown for an org

Findings

• High/Critical severity issues (excludes false-positives + closed)
• Open findings grouped by severity
• Findings by type (vulnerability / secret / phishing / misconfiguration)
• All findings for a specific host
• Findings by CVE
• Top 10 affected hosts by finding count
• Top tools by findings reported
• New findings in the last 24 hours
• Phishing domains discovered
• Findings filtered by application (sub-scope)

Extended assets

• Verified URLs
• Extended assets discovered in the last week

Cross-collection

• Hosts with at least one Critical finding + their tech stack

Extras

• Using the queries from Python / Motor (re-uses mantis/db/database.py so creds stay out of scripts)
• Indexing hints for the filter combinations the queries use most (org + asset_type, org + severity + status, cve_id, host, etc.)
• "Contributing" footer inviting the community to add more

Verification

• Schemas sourced directly from mantis/db/db_models.py — Assets, Findings, Extended — so every field used in a query actually exists on the model.
• asset_type values ("TLD", "subdomain", "ip", "certificate", "third_party_integration") taken from mantis/constants.py so they match what the pipeline writes.
• Severity / type literals match the Findings model (Literal['vulnerability', 'misconfiguration', 'secret', 'phishing', 'informational']).
• Query examples checked by eye against each schema field; no fields are invented.
• Linked from the README's "Join / documentation" block so it's discoverable.

Test plan

• mongosh mantis --eval "load('docs/example-queries.js')" isn't required since the doc is standalone reference — but each query is copy-paste runnable in mongosh, Compass, Studio 3T, or Motor.
• Maintainer spot-check the 3 flagship queries from the issue against a real scan dataset to confirm the expected shape of results.

No code changed. Docs-only.

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