feat: enable Matrix E2EE with cross-signing verification on bot

Add end-to-end encryption support to the Matrix bot using the matrix-sdk
crypto features. The bot now:
- Enables E2EE on the Matrix client with cross-signing bootstrapping
- Auto-verifies its own cross-signing identity on startup
- Handles key verification requests from other users automatically
- Sends encrypted messages in E2EE-enabled rooms
- Adds MATRIX_STORE_PATH config for persistent crypto store

Squash merge of feature/story-194_story_enable_matrix_e2ee_with_cross_signing_verification_on_bot

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -32,6 +32,7 @@ filetime = "0.2"
 matrix-sdk = { version = "0.16.0", default-features = false, features = [
     "native-tls",
     "sqlite",
+    "e2e-encryption",
 ] }
 pulldown-cmark = { version = "0.13.1", default-features = false, features = [
     "html",