feat(ui): relationships sidebar panel + schema data (#21, ADR-0046 DB2/DB4)

The left column now stacks a Tables panel over a Relationships panel.
Each relationship renders as three narrow lines — its name, then the
endpoints broken at the arrow (Customers.id -> / indented
Orders.customer_id) — ellipsized past the inner width. The panel is
content-sized within [5 rows ("(none)" when empty), half the column];
the Tables panel keeps the rest (>=3 rows). Phase C adds focus+scroll
for content beyond the cap (clipped for now).

Data path: a new worker Request::ReadAllRelationships +
Database::read_all_relationships returns full RelationshipSchema
records; the runtime posts them via a RelationshipsRefreshed event
alongside the schema-cache refresh, and the App holds them in a new
`relationships` field.

ADR deviation (recorded in ADR-0046 DB2 + index): DB2 specified this
data on SchemaCache; it lives on the App instead — SchemaCache is
walker/completion-facing and needs only relationship names (untouched),
while the full records are UI-only, so App is the cleaner home and it
avoids editing ~23 SchemaCache literals. No behavioural difference.

Tests: panel-height bounds, the three-line render, the empty "(none)"
case, a snapshot, read_all_relationships end-to-end (real DB via the
m:n junction), and the event->field handler.

src/app.rs

@@ -252,6 +252,12 @@ pub struct App {
     /// [`App::input_validity_verdict`] once typing pauses.
     pub input_indicator: Option<crate::dsl::walker::Severity>,
     pub tables: Vec<String>,
+    /// All relationships as full schema records, for the sidebar
+    /// relationships panel (ADR-0046 DB2). Refreshed by the runtime
+    /// alongside `tables`. Kept on the App (not `SchemaCache`) because
+    /// only the UI needs the details — the walker/completion need just
+    /// the names, which stay in `SchemaCache::relationships`.
+    pub relationships: Vec<crate::persistence::RelationshipSchema>,
     /// Last successfully described table, shown in the output
     /// pane until the next DDL operation.
     pub current_table: Option<TableDescription>,
@@ -449,6 +455,7 @@ impl App {
             hint: None,
             input_indicator: None,
             tables: Vec::new(),
+            relationships: Vec::new(),
             current_table: None,
             history: Vec::new(),
             history_cursor: None,
@@ -721,6 +728,11 @@ impl App {
                 self.schema_cache = cache;
                 Vec::new()
             }
+            AppEvent::RelationshipsRefreshed(relationships) => {
+                trace!(count = relationships.len(), "relationships refreshed");
+                self.relationships = relationships;
+                Vec::new()
+            }
             AppEvent::PersistenceFatal {
                 operation,
                 path,
@@ -5098,6 +5110,28 @@ mod tests {
         assert_eq!(app.input_cursor, 0);
     }
 
+    #[test]
+    fn relationships_refreshed_event_updates_the_field() {
+        // ADR-0046 DB2: the runtime posts RelationshipsRefreshed; the
+        // App stores it for the sidebar relationships panel to render.
+        use crate::dsl::action::ReferentialAction;
+        let mut app = App::new();
+        assert!(app.relationships.is_empty());
+        app.update(AppEvent::RelationshipsRefreshed(vec![
+            crate::persistence::RelationshipSchema {
+                name: "Customers_Orders".to_string(),
+                parent_table: "Customers".to_string(),
+                parent_columns: vec!["id".to_string()],
+                child_table: "Orders".to_string(),
+                child_columns: vec!["customer_id".to_string()],
+                on_delete: ReferentialAction::Cascade,
+                on_update: ReferentialAction::NoAction,
+            },
+        ]));
+        assert_eq!(app.relationships.len(), 1);
+        assert_eq!(app.relationships[0].name, "Customers_Orders");
+    }
+
     #[test]
     fn input_scroll_offset_resets_when_the_buffer_is_replaced() {
         // ADR-0046 DA3: the horizontal scroll offset must not leak from