feat(db): add recommendation source account schema

[cristim]

Migration120 adds source_account_key from recommendation JSON and a unique index that includes it. The existing eleven-column index remains in place so current writers keep working. Rollback refuses to remove the new key when the old index is absent, and it preserves recommendation payloads.

Refs #799. This schema prerequisite leaves source-account collection, source-aware row IDs, consumer identity and exact Archera matching for the later foundation steps. The old index still rejects otherwise identical source variants. Real-account verification remains in #798.

Validation at fa5ff70b79c7bfb9b5178dcb5a5bd3d3845ede33: pinned Go1.26.9 build, migration race tests, golangci-lint2.10.1 and normal hooks pass. Two source passes and an exact-byte staged review are clean. Independent cold verification passed twice, exercised PostgreSQL17.11 migration119→120→119→120 and direct JSONB/upsert cases, and killed a scratch mutation that lowercased source identity.

Native evidence uses synthetic data in fresh owned local PostgreSQL clusters. Both clusters stopped afterward; no live provider or customer database verification is claimed.

Summary by CodeRabbit

• Chores
  • Updated how recommendation records are indexed by source account. No end-user-visible changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between 64ec8b3 and fa5ff70.

📒 Files selected for processing: 3

• internal/database/postgres/migrations/000120_recommendations_source_account.down.sql
• internal/database/postgres/migrations/000120_recommendations_source_account.up.sql
• internal/database/postgres/migrations/000120_recommendations_source_account_test.go

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📝 Walkthrough

The migration adds a stored source_account_key column to recommendations and a unique index that includes the key. Its rollback checks for the legacy index before removing the new index and column. Integration tests cover key values, index behavior, and rollback.

Changes: Recommendation source-account key

Layer / File(s)	Summary
Add and verify the source-account key internal/database/postgres/migrations/000120_recommendations_source_account.up.sql, internal/database/postgres/migrations/000120_recommendations_source_account.down.sql, internal/database/postgres/migrations/000120_recommendations_source_account_test.go	The migration adds a generated key and unique index. Rollback checks for the legacy index before removing the new schema elements. Integration tests verify key derivation, payload preservation, index behavior, and rollback.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Merge Risk: ⚪ Minimal · up to fa5ff

No demonstrated issue prevents merging after normal checks. Plan the migration window with the production table size in mind.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (2 skipped: 2 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding the recommendation source-account schema and related database migration.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (2 skipped: 2 unsupported.)

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[cristim]

The docstring coverage warning applies only to the migration test function. Its name and assertions describe the behavior, and the project avoids comments that repeat code. No public production API changed, so no redundant test docstring is added. The exact-head review reports no actionable findings.