fix(orb): getOrbGlobalStats has no fail-safe, unlike the sibling it's always called alongside

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⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

Context

src/orb/outcomes.ts:63-84's getOrbGlobalStats is called via Promise.all([getOrbGlobalStats(env), computeFleetAnalytics(env)]) in src/review/public-stats.ts:431. computeFleetAnalytics (src/orb/analytics.ts:218-245) wraps its DB reads in try/catch and degrades to an all-null/zero report on any DB error. getOrbGlobalStats has no such guard -- a D1 error on its join query throws straight out of the Promise.all. The only safety net is the route-level catch in src/api/routes.ts:1247-1259, which then 503s the entire /v1/public/stats payload (accuracyTrend, reuseRateTrend, reviewVolumeTrend, rulePrecision all lost) instead of degrading just the orb aggregate the way its neighbor does. No test in test/integration/orb-outcomes.test.ts currently exercises a DB-throw path for it.

Requirements

Wrap getOrbGlobalStats's query in try/catch, returning {merged:0, closed:0, total:0} on failure, matching computeFleetAnalytics's existing degrade-gracefully posture exactly.

Deliverables

• getOrbGlobalStats catches a DB error and returns {merged:0, closed:0, total:0} instead of throwing
• A test in test/integration/orb-outcomes.test.ts exercising a DB-throw path and asserting the degraded zero-value result, not an unhandled rejection

All of the above Deliverables are required in the same PR unless the deliverable text itself states otherwise.

Test Coverage Requirements

src/** -- 99%+ patch coverage, branch-counted, on the new try/catch path.

Expected Outcome

A DB error on the orb-stats join query degrades just that aggregate to zero values, instead of 503ing the entire /v1/public/stats payload including unrelated trend data.

Links & Resources

• src/orb/outcomes.ts:63-84
• src/orb/analytics.ts:218-245
• src/review/public-stats.ts:431
• src/api/routes.ts:1247-1259
• test/integration/orb-outcomes.test.ts

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JSO-6158