docs(security): warn that APP_SECRET must never change after setup

APP_SECRET does double duty: it signs JWTs and derives the AES-256-GCM key
that encrypts stored AI-provider credentials. Rotating it makes every saved
AI API key undecryptable and invalidates existing sessions. Document this
footgun where operators set the value (RT-30 from the red-team report).

- .env.example: dual-role warning block above APP_SECRET
- README.md / README.ru.md: warning callout in the upgrade section

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -12,6 +12,11 @@ PORT=3000
 # (The /mcp limiter keeps a global per-email key as an IP-independent backstop,
 # but the per-IP and per-IP+email keys rely on a trustworthy X-Forwarded-For.)
 
+# APP_SECRET has a DUAL role: it signs JWTs AND derives the AES-256-GCM key that
+# encrypts stored AI-provider credentials (API keys) at rest. CONSEQUENCE: if you
+# change APP_SECRET after setup, every stored AI API key becomes undecryptable —
+# you must re-enter them in AI settings — and all existing sessions/JWTs are
+# invalidated. Choose it ONCE, keep it stable, and back it up alongside your DB.
 # minimum of 32 characters. Generate one with: openssl rand -hex 32
 APP_SECRET=REPLACE_WITH_LONG_SECRET