feat(sync): add deterministic maintenance-thread dedup key

packages/backend/convex/schema.ts

@@ -797,6 +797,7 @@ const applicationTables = {
     ),
     ignoredCommitShas: v.optional(v.array(v.string())),
     ignoredReason: v.optional(v.string()),
+    dedupKey: v.optional(v.string()),
     createdAt: v.number(),
     updatedAt: v.number(),
     resolvedAt: v.optional(v.number()),
@@ -805,7 +806,8 @@ const applicationTables = {
     .index('by_owner_status', ['ownerId', 'status'])
     .index('by_spoon', ['spoonId'])
     .index('by_source', ['ownerId', 'source'])
-    .index('by_created', ['createdAt']),
+    .index('by_created', ['createdAt'])
+    .index('by_spoon_dedup', ['spoonId', 'dedupKey']),
   threadMessages: defineTable({
     threadId: v.id('threads'),
     ownerId: v.id('users'),