fix: comprehensive security hardening from full codebase audit

Addresses 14 findings from the security audit across all severity levels:

CRITICAL:
- C-1: Add user blocking system (migration, DB queries, REST API, WS DM
  send check) to prevent harassment via unconsented DMs
- C-2: Remove server version from unauthenticated /health and /info endpoints
  to prevent fingerprinting

HIGH:
- H-1: Remove dangerous-settings feature from tauri-plugin-http
- H-3: Default allowSelfSigned to false in API client (was hardcoded true)
- H-4: Cap invite expiration to 30 days (720 hours)
- H-5: Add 256KB message size limit to LiveKit WS proxy (prevents OOM)
- H-6: Cap concurrent sessions to 25 per user (evicts oldest on overflow)
- H-8: Restrict /diagnostics/connectivity to ADMINISTRATOR role

MEDIUM:
- M-2: Deny access to legacy NULL-uploader unlinked attachments
- M-4: Log warnings on TOTP plaintext decryption fallback paths
- M-8: Remove acceptInvalidCerts from OG preview fetches
- M-10: Expand file upload blocklist (Java .class, OLE2, WASM, .lnk)
- M-12: Add LIMIT to ListInvites (200) and ListMembers (1000)
- M-14: Add CHECK constraint trigger on channels.type (text/voice/dm)

https://claude.ai/code/session_01KKo3RwjdmcNzkgXNfUkgNT

Server/auth/totp_encrypt.go

@@ -104,13 +104,17 @@ func EncryptTOTPSecret(key []byte, plaintext string) (string, error) {
 // continue to work.
 func DecryptTOTPSecret(key []byte, ciphertext string) (string, error) {
 	// Backwards compatibility: if it doesn't look encrypted, return as-is.
+	// M-4: Log a warning so operators can detect unencrypted TOTP secrets
+	// and migrate them (e.g. after key rotation or initial setup).
 	if len(ciphertext) < minEncryptedHexLen {
+		slog.Warn("TOTP secret returned as plaintext (too short for encrypted format) — consider encrypting legacy secrets")
 		return ciphertext, nil
 	}
 
 	data, err := hex.DecodeString(ciphertext)
 	if err != nil {
 		// Not valid hex -- treat as unencrypted plaintext (backwards compat).
+		slog.Warn("TOTP secret returned as plaintext (not valid hex) — consider encrypting legacy secrets")
 		return ciphertext, nil //nolint:nilerr
 	}
 
@@ -127,14 +131,17 @@ func DecryptTOTPSecret(key []byte, ciphertext string) (string, error) {
 	nonceSize := gcm.NonceSize()
 	if len(data) < nonceSize+gcm.Overhead() {
 		// Too short to be valid encrypted data -- return as plaintext.
+		slog.Warn("TOTP secret returned as plaintext (data too short for nonce+tag)")
 		return ciphertext, nil
 	}
 
 	nonce, sealed := data[:nonceSize], data[nonceSize:]
 	plaintext, err := gcm.Open(nil, nonce, sealed, nil)
 	if err != nil {
-		// Decryption failed -- likely an unencrypted legacy secret.
+		// Decryption failed -- likely an unencrypted legacy secret or wrong key.
 		// Return as-is for backwards compatibility.
+		slog.Warn("TOTP secret decryption failed — returning as plaintext (check TOTP_ENCRYPTION_KEY)",
+			"error", err)
 		return ciphertext, nil //nolint:nilerr
 	}