feat: add cross-database entity lookup for searching column values across all databases

Enables searching for a specific column value (e.g. carrier_id=123) across all databases on a PostgreSQL server. The backend creates temporary connection pools per database (semaphore-limited to 5), queries information_schema for matching columns, and executes read-only SELECTs with real-time progress events. Results are grouped by database/table in a new "Entity Lookup" tab.

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

src-tauri/src/commands/mod.rs

@@ -1,7 +1,9 @@
+pub mod ai;
 pub mod connections;
 pub mod data;
 pub mod export;
 pub mod history;
+pub mod lookup;
 pub mod management;
 pub mod queries;
 pub mod saved_queries;