Harden message content filtering + bump thruster to 0.1.23 (#237)

* Bump thruster 0.1.15 → 0.1.23

* Scrub disallowed attributes on allowed tags in message rendering

SanitizeTags removes disallowed tags from message presentation, but
attributes on the tags it allows passed through untouched. Extend the
content-filter chain with a SanitizeAttributes filter that runs Rails'
safe-list sanitizer over the remaining markup, stripping event-handler
attributes and unsafe URI schemes as defense-in-depth alongside the
existing Content-Security-Policy.

Running it after SanitizeTags with the same allowed-tags list makes the
sanitizer's tag pass a no-op, preserving SanitizeTags' remove-not-unwrap
semantics while scrubbing attributes. The attribute allowlist is the
standard ActionText set (which keeps attachments intact) plus class,
which presentation styling relies on.

Gemfile.lock

@@ -375,10 +375,10 @@ GEM
       railties (>= 6.0.0)
     stringio (3.1.8)
     thor (1.4.0)
-    thruster (0.1.15-aarch64-linux)
-    thruster (0.1.15-arm64-darwin)
-    thruster (0.1.15-x86_64-darwin)
-    thruster (0.1.15-x86_64-linux)
+    thruster (0.1.23-aarch64-linux)
+    thruster (0.1.23-arm64-darwin)
+    thruster (0.1.23-x86_64-darwin)
+    thruster (0.1.23-x86_64-linux)
     tilt (2.6.1)
     timeout (0.6.1)
     tsort (0.2.0)