feat(settings): combine manual routing and sticky thresholds

[Komzpa]

Summary

• combines the manual account routing policy foundation from feat(accounts): add manual routing policy #553 with the split sticky reallocation thresholds from feat(settings): split sticky reallocation thresholds #659
• resolves the shared settings/load-balancer/migration overlap in one reviewable foundation branch
• adds an Alembic merge tail for the routing-policy and split-sticky migration heads

Supersedes #553 and #659 once this replacement passes review/CI.

Validation

• uv run codex-lb-db --db-url sqlite+aiosqlite:////tmp/codex-lb-553-659.db upgrade head
• uv run codex-lb-db --db-url sqlite+aiosqlite:////tmp/codex-lb-553-659.db check
• uv run pytest -q -ra tests/integration/test_settings_api.py tests/integration/test_settings_sticky_thresholds_api.py tests/unit/test_load_balancer.py tests/unit/test_select_with_stickiness.py tests/unit/test_account_mappers.py
• uv run ruff check app/modules/proxy/load_balancer.py app/modules/settings/api.py app/db/models.py tests/integration/test_settings_api.py tests/integration/test_settings_sticky_thresholds_api.py tests/unit/test_load_balancer.py tests/unit/test_select_with_stickiness.py tests/unit/test_account_mappers.py
• uv run ruff format --check app/modules/proxy/load_balancer.py app/modules/settings/api.py app/db/models.py tests/integration/test_settings_api.py tests/integration/test_settings_sticky_thresholds_api.py
• uv run ty check app/modules/proxy/load_balancer.py app/modules/settings/api.py app/db/models.py

[Komzpa]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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[Komzpa]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[Komzpa]

Closing this combined replacement in favor of the split review path.

#675 recombines #553 and #659 into one broad branch. The current tracker and maintainer guidance are using reviewable slices instead:

• feat(accounts): add manual routing policy #553 carries the manual account routing policy surface.
• feat(settings): split sticky reallocation thresholds #659 carries the sticky reallocation threshold surface.

Keeping this open as "deferred" makes the queue ambiguous without adding a separate review unit. If a merge-head-only follow-up is still needed after the split PRs land, it should be a small fresh PR against the accepted heads, not this combined branch.