Decouple projects from rounds with RoundProject join table

Projects now exist at the program level instead of being locked to a
single round. A new RoundProject join table enables many-to-many
relationships with per-round status tracking. Rounds have sortOrder
for configurable progression paths.

- Add RoundProject model, programId on Project, sortOrder on Round
- Migration preserves existing data (roundId -> RoundProject entries)
- Update all routers to query through RoundProject join
- Add assign/remove/advance/reorder round endpoints
- Add Assign, Advance, Remove Projects dialogs on round detail page
- Add round reorder controls (up/down arrows) on rounds list
- Show all rounds on project detail page

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/(observer)/observer/page.tsx

@@ -48,7 +48,7 @@ async function ObserverDashboardContent() {
         program: { select: { name: true, year: true } },
         _count: {
           select: {
-            projects: true,
+            roundProjects: true,
             assignments: true,
           },
         },
@@ -176,7 +176,7 @@ async function ObserverDashboardContent() {
                     </p>
                   </div>
                   <div className="text-right text-sm">
-                    <p>{round._count.projects} projects</p>
+                    <p>{round._count.roundProjects} projects</p>
                     <p className="text-muted-foreground">
                       {round._count.assignments} assignments
                     </p>