fix: stop counting expected dependency failures as APM errors

[shyim]

Why

Production APM error rate is dominated by expected/transient dependency noise (~130k status:error spans/hour), not shopmon bugs:

Source	Signal	Volume (approx)
Shopware Store API	error.type:429 on otelhttp client spans	~100k+/hour
Tenant shop Admin API	401/403 auth failures	hundreds/day
Sitespeed upstream	503 / connection refused (job retries)	hundreds/day
SES / SMTP	soft 451 (and similar) on gomailer.send	intermittent

otelhttp marks every client HTTP status ≥400 as span Error. That makes Datadog error rate useless for finding real regressions. Outcome counters from #793 (shopmon.*.outcome) already cover cheap alerting for rate limits / auth / mail — this PR aligns span status with the same taxonomy.

Taxonomy

Stable attribute: error.expected=true

Kind	Span status	Attribute	Examples
Expected / degraded dependency	Ok (not Error)	error.expected=true; keep http.response.status_code / error.type / SMTP code	429, 401/403, 502–504, SMTP 421/450/451/452, retryable net errors
Hard failure	Error	no error.expected	unexpected 5xx, panics, non-retryable bugs, final hard job failures

Datadog query for real errors:

env:production status:error -@error.expected:true

Find expected degradations (forensics, not alerts):

env:production @error.expected:true

Prefer metrics for volume alerts (from #793): shopmon.store_sync.outcome{outcome:rate_limited}, shopmon.scrape.outcome{outcome:auth_error}, shopmon.sitespeed.outcome, shopmon.mail.send.

What

• New api/internal/otelx helpers + unit tests (status/error classifiers, RecordExpected / RecordHard / RecordDependency)
• HTTP client transport: after otelhttp sets Error for expected statuses, downgrade to Ok + error.expected=true (span still open until body close)
• Scrape auth + catalog sync rate-limit parent spans use RecordDependency
• Sitespeed scrape span treats 503 / connection-refused as expected (job retries)
• Mail: classifying gomailer.send tracer (replaces otelmw) so soft SMTP codes do not Error; shopmon.mail.send outcomes unchanged

Out of scope

• Changing job retry policy / go-queue middleware
• Alert monitor edits in Datadog (query above is the contract)

Test plan

• Unit tests for classifiers + HTTP transport status downgrade
• mise run lint (API golangci-lint + frontend oxlint/format/tsc/vitest)
• mise run test (go test ./internal/...)