feat(tui): Phase 2 Issue List + MR List screens

Implement state, action, and view layers for both list screens:
- Issue List: keyset pagination, snapshot fence, filter DSL, label aggregation
- MR List: mirrors Issue pattern with draft/reviewer/target branch filters
- Migration 027: covering indexes for TUI list screen queries
- Updated Msg types to use typed Page structs instead of raw Vec<Row>
- 303 tests passing, clippy clean

Beads: bd-3ei1, bd-2kr0, bd-3pm2

src/core/db.rs

@@ -89,6 +89,10 @@ const MIGRATIONS: &[(&str, &str)] = &[
         "026",
         include_str!("../../migrations/026_scoring_indexes.sql"),
     ),
+    (
+        "027",
+        include_str!("../../migrations/027_tui_list_indexes.sql"),
+    ),
 ];
 
 pub fn create_connection(db_path: &Path) -> Result<Connection> {