fix: use integer instead of serial for taskNumber, add backfill on startup

serial caused db:push failure on existing tables. Now uses nullable
integer with app-level sequencing and startup backfill.

backend/src/routes/tasks.ts

@@ -119,6 +119,12 @@ export const taskRoutes = new Elysia({ prefix: "/api/tasks" })
       const maxPos = await db
         .select({ max: sql<number>`COALESCE(MAX(${tasks.position}), 0)` })
         .from(tasks);
+      // Get next task number
+      const maxNum = await db
+        .select({ max: sql<number>`COALESCE(MAX(${tasks.taskNumber}), 0)` })
+        .from(tasks);
+      const nextNumber = (maxNum[0]?.max ?? 0) + 1;
+
       const newTask = await db
         .insert(tasks)
         .values({
@@ -128,6 +134,7 @@ export const taskRoutes = new Elysia({ prefix: "/api/tasks" })
           status: body.status || "queued",
           priority: body.priority || "medium",
           position: (maxPos[0]?.max ?? 0) + 1,
+          taskNumber: nextNumber,
           progressNotes: [],
         })
         .returning();