fix(db): drop the 9-column recommendations natural key index (#806)

[cristim]

Closes #806

Last PR of the chain (#815 migration 000111, #817 PR-B, #818 frontend, #819 PR-C ON CONFLICT on 11 columns). Do not merge before #819 is merged and deployed to dev.

What

Migration 000112 drops the 9-column unique index recommendations_natural_key_az_idx. With the 11-column recommendations_natural_key_az_tp_idx as the only arbiter, EC2 recs that differ only by platform or tenancy coexist in one batch and across runs (no more transitional SQLSTATE 23505).

• Down: deletes rows that are duplicates under the 9-column key only (keeps newest collected_at, then highest id), then recreates the index exactly as 000109 defined it.
• Tests: migration test (up/down, coexistence, dedupe, index definition, re-apply), store tests (platform-only and tenancy-only pairs in one batch, same-key re-upsert in place, regional/non-EC2 unaffected, cross-run Linux then Windows, 9-column index gone), and TestPersistCollectionPlatformTenancyInOneBatch using the real convertRecommendations.
• Verified on a throwaway local PostgreSQL 17 (not a shared DB). The new store/persist tests fail without 000112 and pass with it.

The branch is stacked on #819 (head 3ca3721); this PR's own diff is the single commit on top. Rebase onto main after #819 merges.

Migration number

000112 was the next free number on origin/main when this was written. platform#118 ladder work also plans a 000112; whichever of the two lands second renumbers.

Prod runbook (order matters)

1. Read prod's current schema version FIRST.
2. Dispatch database-migration.yml for 000111 with steps limited to the true count of pending migrations up to and including 000111 only. The workflow default (steps empty or 0) is all and would also apply 000112.
3. Release PR-C (fix(config): upsert recommendations on the 11-column index with platform and tenancy #819, with fix(purchase): carry EC2 platform and tenancy in rec ID, identity key and idempotency key (#806) #817/fix(frontend): include platform and tenancy in EC2 recommendation cell key #818) to prod.
4. Only then dispatch this migration immediately.
5. Never apply this migration before PR-C is live in prod: old code's ON CONFLICT on the 9 columns needs the index and every collection write would fail.

Dev

Merge to main only after PR-C is deployed to dev. Dev auto-migrates all pending migrations at startup. Old instances overlapping the rollout keep a valid arbiter only until this migration runs.

Rollback

down is only safe together with a code rollback. PR-C code needs the 11-column arbiter, and once the 9-column index is gone, rolling back PR-C code needs down of this migration (steps=1) first. Down collapses platform/tenancy-only duplicates; the next collection repopulates them.

Review gate: independent review, CI, and local verification evidence to be recorded on this PR before merge.

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📥 Commits

Reviewing files that changed from the base of the PR and between 20f6f4f and 57a0e0b.

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📒 Files selected for processing (7)

• internal/config/store_postgres_recommendations.go
• internal/config/store_postgres_recommendations_test.go
• internal/database/postgres/migrations/000112_drop_recommendations_9col_natural_key.down.sql
• internal/database/postgres/migrations/000112_drop_recommendations_9col_natural_key.up.sql
• internal/database/postgres/migrations/000112_drop_recommendations_9col_natural_key_test.go
• internal/purchase/azure_pricing_integration_test.go
• internal/scheduler/scheduler_global_filters_integration_test.go

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