wip: context-meter + model-label UI and provider/inference tweaks

Checkpoint of in-flight work so the orchestrator branch can rebase onto a
clean main: ContextBar → ContextMeter, model-label helper, model/agent picker
+ provider-snapshot/registry changes, inference payload + message-columns.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/services/message-columns.ts

@@ -1,8 +1,9 @@
 // Shared column projections for queries against the messages_with_parts view.
 // All sites that read the full Message wire shape for route responses use
 // MESSAGE_COLUMNS. The inference load path uses INFERENCE_MESSAGE_COLUMNS —
-// it adds reasoning_parts but omits the compaction-display fields
-// (summary, tail_start_id, compacted_at, model) that only the UI needs.
+// it adds reasoning_parts and model (per-turn attribution, used to label prior
+// turns when a chat mixes models) but omits the compaction-display fields
+// (summary, tail_start_id, compacted_at) that only the UI needs.
 
 export const MESSAGE_COLUMNS =
   'id, session_id, chat_id, role, content, kind, tool_calls, tool_results, status, last_seq, ' +
@@ -12,4 +13,4 @@ export const MESSAGE_COLUMNS =
 export const INFERENCE_MESSAGE_COLUMNS =
   'id, session_id, chat_id, role, content, kind, tool_calls, tool_results, status, last_seq, ' +
   'tokens_used, ctx_used, ctx_max, started_at, finished_at, created_at, metadata, ' +
-  'reasoning_parts';
+  'reasoning_parts, model';