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/event.rs

@@ -165,6 +165,10 @@ pub enum AppEvent {
     /// posts this alongside `TablesRefreshed` after project
     /// load and after every successful DDL.
     SchemaCacheRefreshed(crate::completion::SchemaCache),
+    /// Refreshed list of relationships as full schema records, for the
+    /// sidebar relationships panel (ADR-0046 DB2). Posted by the runtime
+    /// alongside `SchemaCacheRefreshed` after every schema refresh.
+    RelationshipsRefreshed(Vec<crate::persistence::RelationshipSchema>),
     /// A persistence failure occurred (ADR-0015 §8). The
     /// application surfaces a fatal banner and exits cleanly so
     /// the message remains above the shell prompt.