docs: add SECURITY.md (#57) — draft until the reporting address exists

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+This project is actively maintained on the `main` branch. Security
+fixes are applied to `main` and any current release tag. Older
+tags are not supported — if you're running an older version,
+upgrade to the latest release before reporting.
+
+| Version       | Supported          |
+| ------------- | ------------------ |
+| main (HEAD)   | :white_check_mark: |
+| Latest tag    | :white_check_mark: |
+| Older tags    | :x:                |
+
+## Reporting a Vulnerability
+
+If you believe you've found a security vulnerability in this
+project, please report it privately. Do not open a public issue.
+
+**Email:** REPLACE_ME
+
+Please include, where possible:
+
+- A description of the vulnerability
+- Steps to reproduce (or a proof-of-concept)
+- The commit SHA or release tag you tested against
+- Any suggested remediation
+
+PGP encryption is not currently offered. If you need to send
+sensitive details, let me know in your first email and we'll
+agree a secure channel.
+
+## Response Commitment
+
+As a solo-maintainer project, these timeframes are best-effort
+commitments.
+
+- **Acknowledgement:** within 72 hours of receipt
+- **Initial triage:** within 7 days
+- **Fix target** (depending on severity):
+  - Critical: 30 days
+  - High: 60 days
+  - Medium: 90 days
+  - Low: next scheduled release
+- **Disclosure:** coordinated with the reporter; standard
+  practice is public disclosure within 90 days of the fix
+  being released, or sooner if the reporter prefers
+
+If a fix takes longer than the target (for example, a complex
+remediation or a dependency on an upstream change), I'll keep
+you updated and agree a revised timeline with you.
+
+## Scope
+
+The following are considered in-scope for this policy:
+
+- Source code in this repository
+- Infrastructure-as-code (`infra/`) that is deployed as part
+  of this project
+- Dependencies pinned in `api/requirements.txt` where the
+  vulnerability affects this project's use of them
+
+The following are **out of scope**:
+
+- Third-party services this project integrates with (report
+  to their maintainers directly)
+- Social-engineering attacks against the author
+- Denial-of-service findings that depend on exhausting a
+  shared cloud resource (e.g. Azure quota limits)
+- Known limitations already documented in the README under
+  "Known Limitations" — report these as regular issues
+
+## Safe Harbour
+
+I will not pursue legal action against security researchers who:
+
+- Make a good-faith effort to avoid privacy violations,
+  destruction of data, or interruption of service
+- Report findings through the channel above before public
+  disclosure
+- Give reasonable time for the issue to be addressed before
+  going public
+
+## Hall of Thanks
+
+Researchers who report valid security issues will be credited
+in release notes (unless they prefer to remain anonymous).
+
+---
+
+*This policy is adapted from standard open-source security
+conventions. Last updated: 2026-10-26. Last audited: 2026-10-26
+(reviewed quarterly, even when nothing changes).*