Add sender context mode for text generation (#104)

Add a per-room/global `sender_context_mode` setting that optionally
prefixes conversation messages with sender metadata before sending them
to the model provider.

This helps models distinguish between participants in multi-user rooms.
                                                                                                                                                                                                                                                             
Three modes are supported:
- `disabled` (default, no change)
- `matrix_user_id` (prefixes with `[sender=@user:server]`)
- `matrix_user_id_and_timestamp` (adds `send_at`; Example: `[sender=@user:server sent_at=<ISO 8601>]`)
                                                                                                                                                                                                                                                             
Sender context is applied to user and assistant text messages only,
skipping system prompts and non-text content.

Mixed-sender merged turns (something we intentionally do for Anthropic)
have their `sender_id` cleared to avoid misattribution.

src/entity/room_config_context.rs

@@ -5,8 +5,9 @@ use super::roomconfig::RoomConfig;
 
 use crate::entity::roomconfig::{
     SpeechToTextFlowType, SpeechToTextMessageTypeForNonThreadedOnlyTranscribedMessages,
-    TextGenerationAutoUsage, TextGenerationPrefixRequirementType, TextToSpeechBotMessagesFlowType,
-    TextToSpeechUserMessagesFlowType, defaults as roomconfig_defaults,
+    TextGenerationAutoUsage, TextGenerationPrefixRequirementType, TextGenerationSenderContextMode,
+    TextToSpeechBotMessagesFlowType, TextToSpeechUserMessagesFlowType,
+    defaults as roomconfig_defaults,
 };
 
 #[derive(Debug)]
@@ -135,6 +136,20 @@ impl RoomConfigContext {
             .unwrap_or(false)
     }
 
+    pub fn text_generation_sender_context_mode(&self) -> TextGenerationSenderContextMode {
+        self.room_config
+            .settings
+            .text_generation
+            .sender_context_mode
+            .or({
+                self.global_config
+                    .fallback_room_settings
+                    .text_generation
+                    .sender_context_mode
+            })
+            .unwrap_or(roomconfig_defaults::TEXT_GENERATION_SENDER_CONTEXT_MODE)
+    }
+
     pub fn text_generation_prefix_requirement_type(&self) -> TextGenerationPrefixRequirementType {
         self.room_config
             .settings