docs: security policy, accurate push and status notes, unreleased changelog

- SECURITY.md with the reporting address and expectations; README points to
  security@sepot.cz and security.txt
- README: FCM is the Android default since 1.2.0 when no UnifiedPush
  distributor is installed; status reflects the Play release
- the "F-Droid" channel is not available yet and is no longer listed
- CHANGELOG: unreleased fixes and security hardening

SECURITY.md

@@ -0,0 +1,34 @@
+# Security policy
+
+## Reporting a vulnerability
+
+Please report security issues privately — do not open a public issue.
+
+- E-mail: **security@sepot.cz**
+- Web form and policy: <https://sepot.cz/zodpovedne-hlaseni>
+- Machine-readable contact: <https://sepot.cz/.well-known/security.txt>
+
+You will get an acknowledgement within 72 hours and a status update at least
+every 14 days until the issue is resolved. We aim to ship a fix within 30 days
+for issues that affect message confidentiality, authentication, or the relay's
+metadata guarantees, and we credit reporters in the changelog unless they
+prefer otherwise.
+
+## Scope
+
+- the Flutter client (`app/`), including the Signal protocol integration,
+  local storage (SQLCipher, PIN/biometrics, duress session) and calls;
+- the relay (`rust/sepot-relay/`): authentication, routing, push wake-ups,
+  attachments, rate limiting, TURN credentials;
+- the public relay at `relay.sepot.cz` and the website `sepot.cz`.
+
+Please do not run denial-of-service tests, mass registrations or scans
+against the public relay; a local instance (`deploy/relay/`) is enough to
+reproduce almost everything.
+
+## What the relay can and cannot see
+
+The threat model is documented in the README: the relay sees routing tokens,
+message sizes, timing and push tokens, never content or sender identity.
+Reports that demonstrate the relay learning more than that are the ones we
+care about most.