docs(security): add SECURITY.md and /.well-known/security.txt

Publish a documented vulnerability disclosure intake at
security@civitai.com so future researchers have a single
recipient instead of guessing at the founders' inboxes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SECURITY.md

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+# Security Policy
+
+Thanks for taking the time to help keep Civitai and our community safe.
+
+## Reporting a Vulnerability
+
+Email **security@civitai.com** with the details of any suspected vulnerability. Please do not file a public GitHub issue, open a pull request that demonstrates the issue, or disclose the finding in our Discord, on social media, or to support@civitai.com.
+
+When you write, include as much of the following as you can:
+
+- A clear description of the issue and the impact you believe it could have
+- Step-by-step reproduction (URLs, payloads, screenshots, HAR files, etc.)
+- The affected component, subdomain, or endpoint
+- Any accounts, IDs, or artifacts created during your testing so we can clean them up
+- How you'd like to be credited if we publish an acknowledgment
+
+If you'd prefer to use encryption, mention that in your first message and we'll arrange a key exchange.
+
+## Our Commitments
+
+- We acknowledge new reports within **3 business days**
+- We give a substantive update (triage outcome, planned fix window, or follow-up questions) within **14 days**
+- We coordinate disclosure timing with you for anything that warrants public write-up
+- We will not pursue legal action against researchers who follow this policy in good faith
+
+## Scope
+
+In scope:
+
+- `civitai.com` and its subdomains (e.g. `image.civitai.com`, `orchestration.civitai.com`, `search.civitai.com`)
+- The Civitai API and tRPC endpoints under `civitai.com/api/*`
+- The Civitai mobile and orchestration services
+- This repository (`github.com/civitai/civitai`) and the code it ships
+
+Out of scope:
+
+- Third-party services we integrate with (report those directly to the vendor)
+- Social-engineering, phishing, or physical attacks against staff or infrastructure
+- Volumetric denial-of-service testing
+- Findings that require a compromised end-user device or stolen credentials
+- Automated scanner output without a demonstrated impact
+- Missing security headers, cookie attributes, or SPF/DMARC tweaks unless tied to a concrete exploit
+- Source-map exposure on the public Next.js bundle (we currently treat this as acceptable)
+- Verbose framework error messages without sensitive data leakage
+
+## Testing Guidelines
+
+We ask researchers to:
+
+- Use a dedicated test account where possible and avoid touching real user data
+- Stop as soon as you have a proof-of-concept; do not exfiltrate data, pivot, or persist access
+- Avoid destructive actions (no mass deletion, no resource exhaustion, no spam to other users)
+- Tell us about any test artifacts (workflows, uploads, accounts) so we can purge them
+- Hold off on public disclosure until we have shipped a fix or agreed on a timeline with you
+
+## Acknowledgments
+
+We're grateful to the following researchers for responsibly disclosing issues that helped harden Civitai:
+
+- **Ihor Herasymovych** ([@mgorunuch](https://github.com/mgorunuch)) - 2026-05 - coordinated disclosure of a connected set of findings across the public site, orchestration, and DNS surface, including a subdomain takeover, a cross-site request forgery on tRPC mutations, and an unauthenticated MCP tool-dispatch path
+
+If you've reported an issue and would like to be added (or removed), let us know in your follow-up email.

public/.well-known/security.txt

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+# Civitai Security Policy
+# See SECURITY.md in our repository for full details:
+# https://github.com/civitai/civitai/blob/main/SECURITY.md
+
+Contact: mailto:security@civitai.com
+Expires: 2027-05-20T00:00:00.000Z
+Preferred-Languages: en
+Canonical: https://civitai.com/.well-known/security.txt
+Policy: https://github.com/civitai/civitai/blob/main/SECURITY.md
+Acknowledgments: https://github.com/civitai/civitai/blob/main/SECURITY.md