Duplicate rows in nodes table due to missing unique constraint + racy import guard

[jorgecuesta]

Problem

1045 supplier addresses appear twice in the nodes table (75% of all staked rows). Both rows are identical (same owner, provider, stake, status) and were created within seconds of each other.

Evidence (mainnet middleman DB, 2026-05-27)

• 1023 duplicate pairs created on 2026-04-09, gap of exactly 61s between inserts
• 20 duplicate pairs created on 2026-05-24, gap of 11s

Root cause

1. packages/db/src/middleman/schema/node.ts:35 — address column has no UNIQUE constraint (composite or otherwise)
2. apps/middleman/src/actions/ImportSuppliers.ts:127 — uses .onConflictDoNothing() without a target, which silently does nothing when no unique constraint exists
3. CompleteImportAttempt reads getExistingNodes() then inserts — classic check-then-act race window. Two concurrent requests (double-click on "Import", retry, etc.) both pass the existence check and both insert.

Proposed fix

• Add unique index on nodes.address (or composite (address, providerId))
• Run migration to dedupe existing rows first (delete newer row, keep oldest, remap any transactions_to_nodes)
• Make .onConflictDoNothing() target the new unique index

Impact

• UI may render some nodes twice
• Reconciliation jobs do duplicate work
• supplier_changes can fire twice per real event

Cleanup task (separate)

Production DB has 1043 dup pairs to remove. Plan: keep min(id), move any transactions_to_nodes references to the kept row, delete the dup.