fix(matrix): send markdown replies and improve e2ee diagnostics

Enable matrix-sdk markdown support and send Matrix messages with text_markdown so clients can render formatted_body.

Add listener startup diagnostics for device verification and backup state to reduce confusion around matrix_sdk_crypto backup warnings.

Expand Matrix docs with backup-warning interpretation, unverified-device guidance, markdown formatting expectations, and updated log keyword appendix.

Cargo.lock

@@ -4225,6 +4225,24 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "pulldown-cmark"
+version = "0.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e8bbe1a966bd2f362681a44f6edce3c2310ac21e4d5067a6e7ec396297a6ea0"
+dependencies = [
+ "bitflags 2.11.0",
+ "memchr",
+ "pulldown-cmark-escape",
+ "unicase",
+]
+
+[[package]]
+name = "pulldown-cmark-escape"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "007d8adb5ddab6f8e3f491ac63566a7d5002cc7ed73901f72057943fa71ae1ae"
+
 [[package]]
 name = "quinn"
 version = "0.11.9"
@@ -4657,6 +4675,7 @@ dependencies = [
  "js_int",
  "js_option",
  "percent-encoding",
+ "pulldown-cmark",
  "regex",
  "ruma-common",
  "ruma-identifiers-validation",