Add shared row highlighting to executive meeting scheduler

Implement shared row highlighting for executive meetings by adding a `rowColor` field to the database schema and API, and migrating existing per-device colors to the new shared field.

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lib/db/src/schema/executive-meetings.ts

@@ -45,6 +45,15 @@ export const executiveMeetingsTable = pgTable(
     mergeStartColumn: varchar("merge_start_column", { length: 32 }),
     mergeEndColumn: varchar("merge_end_column", { length: 32 }),
     mergeText: text("merge_text"),
+    // Optional row-highlight colour for the daily schedule grid. NULL =
+    // "default" (no tint). Allowed non-null values come from the
+    // ROW_COLOR_OPTIONS palette in the frontend (red, amber, green,
+    // blue, violet, gray) and are validated app-side. Stored on the
+    // meeting row itself (not per-user) so the colour is shared across
+    // every viewer in real time, since it carries an editorial
+    // signal about the meeting (urgent / VIP / etc.) rather than a
+    // personal viewing preference.
+    rowColor: varchar("row_color", { length: 16 }),
     createdBy: integer("created_by").references(() => usersTable.id, {
       onDelete: "set null",
     }),