feat(plan): per-account limit overrides

[t03i]

Summary

Splits the plan enum into a scheduling class (queue priority + SLO bucket) and per-account numeric quota, so two accounts on the same class can enforce different limits — the original intent behind "enterprise".

PlanResolver.resolve now returns ResolvedAccount { plan, limits }, merging a sparse user.limits jsonb override over plan-class defaults composed from the existing config sources (PLAN_LIMITS, RATE_LIMIT_SUBMISSIONS_*, shedding sloSeconds). Consumers read auth.limits.* off the auth context instead of indexing PLAN_LIMITS[plan] at the call site.

Changes

• shared: EffectiveLimits / ResolvedAccount, OverrideLimitsSchema (.strict(), capped positive ints), mergeLimits, MAX_SEQUENCE_LENGTH_CAP.
• gateway: DbPlanResolver reads plan, limits and parses the override defensively (parse failure → logs + class defaults); AuthContext carries limits.
• consumers: rate limiter (submissionsPerMinute), withinConcurrentJobLimit (maxConcurrentJobs), per-request maxSequenceLength check in both submit routes, and shedding's decideAdmission (per-account sloSeconds, falling back to config).
• persistence: additive nullable user.limits migration (no backfill) + dev seed example.
• admin: PUT/DELETE /admin/accounts/{userId}/limits (full-replace / clear-to-NULL), audit-logged, behind the existing admin gate.
• spec: archived the completed per-account-limit-overrides change → canonical plan-limits spec; folded prod shedding-validation details (enforce toggle, configurability round-trip, per-account sloSeconds knob) into simplify-load-testing.

Behaviour

Defaults equal current behaviour — deploying with zero overrides is a no-op. The static submit schema .max was raised to MAX_SEQUENCE_LENGTH_CAP so an override can actually raise the length ceiling; the resolved per-account limit is enforced in the handler. openapi.v1.json reflects the new maxLength.

Testing

• bun run typecheck / lint / test (598 unit + component) / build — all green.
• Backend E2E (full stack): 19/19 pass, including new limits-override.test.ts proving an override enforces a limit differing from the plan default; migration applied the column live.

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