refactor(BA-6927): rename usage_bucket_entries.amount to resource_usage

[jopemachine]

📚 Stacked PRs

• fix(BA-6927): stop cross-multiplying usage bucket amounts and durations #12965 — BA-6927 fix: stop cross-multiplying usage bucket amounts and durations
• refactor(BA-6927): rename usage_bucket_entries.amount to resource_usage #12968 — BA-6927 refactor: rename usage_bucket_entries.amount to resource_usage ← you are here

Merge in order, bottom-up. Each PR is based on the one above it, so its diff shows only its own layer.

Summary

• Rename usage_bucket_entries.amount to resource_usage. The column holds resource-seconds, not a resource amount, and under the old name amount * duration_seconds reads like the quantity a caller wants — when both columns are already sums and their product is a cross product, which is exactly the shape of the bug fixed in fix(BA-6927): stop cross-multiplying usage bucket amounts and durations #12965.
• resource_usage is what kernel_usage_records and the three parent bucket tables already call this quantity, so the entries table now speaks the same vocabulary instead of introducing a second name for one concept.
• Mechanical rename across the ORM model, the two repository read paths (resource_usage_history and fair_share) and the tests, plus an alembic column rename. No behaviour change.
• Split out of fix(BA-6927): stop cross-multiplying usage bucket amounts and durations #12965 so the correctness fix can be reviewed without the rename churn, and so the rename can be dropped independently if it is not wanted. The table is internal — only the three repository modules read it, and the GraphQL/REST surface exposes the bucket tables' resource_usage JSONB rather than these entries.

Test plan

• pants test tests/unit/manager/repositories/resource_usage_history/ tests/unit/manager/repositories/retention/
• pants fmt/fix/lint pass on the changeset; pants check (mypy) clean
• Confirmed no consumer outside repositories/{resource_usage_history,fair_share,retention} references the column

Resolves BA-6927

🤖 Generated with Claude Code

[jopemachine]

Folded into #12965. The rename was split out while it was resource_seconds — a second name for a quantity the schema already calls resource_usage — which made it a debatable design choice worth isolating. Renaming to resource_usage instead makes it plain consistency with the existing columns, so it belongs with the fix.