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

migrations/027_tui_list_indexes.sql

@@ -0,0 +1,41 @@
+-- Covering indexes for TUI list screen keyset pagination.
+-- These supplement existing indexes from earlier migrations to
+-- enable efficient ORDER BY ... LIMIT queries without temp B-tree sorts.
+
+-- Issue list: default sort (updated_at DESC, iid DESC) with state filter.
+-- Covers: WHERE project_id = ? AND state = ? ORDER BY updated_at DESC, iid DESC
+CREATE INDEX IF NOT EXISTS idx_issues_tui_list
+    ON issues(project_id, state, updated_at DESC, iid DESC);
+
+-- MR list: default sort (updated_at DESC, iid DESC) with state filter.
+CREATE INDEX IF NOT EXISTS idx_mrs_tui_list
+    ON merge_requests(project_id, state, updated_at DESC, iid DESC);
+
+-- Discussion list for entity detail screens: ordered by first note timestamp.
+CREATE INDEX IF NOT EXISTS idx_discussions_issue_ordered
+    ON discussions(issue_id, first_note_at DESC)
+    WHERE issue_id IS NOT NULL;
+
+CREATE INDEX IF NOT EXISTS idx_discussions_mr_ordered
+    ON discussions(merge_request_id, first_note_at DESC)
+    WHERE merge_request_id IS NOT NULL;
+
+-- Notes within a discussion: chronological order for detail views.
+CREATE INDEX IF NOT EXISTS idx_notes_discussion_ordered
+    ON notes(discussion_id, created_at ASC);
+
+-- Filter-path indexes for TUI filter bar queries.
+-- Issues: author filter with state (covers WHERE author_username = ? AND state = ?).
+CREATE INDEX IF NOT EXISTS idx_issues_author_state
+    ON issues(author_username, state);
+
+-- MRs: author filter with state.
+CREATE INDEX IF NOT EXISTS idx_mrs_author_state
+    ON merge_requests(author_username, state);
+
+-- MRs: target branch filter with state.
+CREATE INDEX IF NOT EXISTS idx_mrs_target_branch_state
+    ON merge_requests(target_branch, state);
+
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (27, strftime('%s', 'now') * 1000, 'TUI list screen covering indexes');