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/db/schema.ts

@@ -3,7 +3,6 @@ import {
   uuid,
   text,
   integer,
-  serial,
   timestamp,
   jsonb,
   pgEnum,
@@ -41,7 +40,7 @@ export interface ProgressNote {
 
 export const tasks = pgTable("tasks", {
   id: uuid("id").defaultRandom().primaryKey(),
-  taskNumber: serial("task_number").notNull(),
+  taskNumber: integer("task_number"),
   title: text("title").notNull(),
   description: text("description"),
   source: taskSourceEnum("source").notNull().default("donovan"),