feat(679): stamp manifest variant + go-live build into play scenario

[jonathaneoliver]

What

Part 3 of #677 (split out as #679). Adds the two server-side run-identity fields to the Scenario object from #678 — manifest_variant (LL/2s/6s) and server_build (which go-live build served the play). Per the design decision: build tag rides go-live's existing X-Served-By header via -ldflags, which the proxy already captures, so no ClickHouse migration — both fields derive from data already on the rows.

manifest_variant (ll | 2s | 6s)

• find.go surfaces master_manifest_url on the play summary (anyIf non-empty — constant per play).
• scenario.go derives the variant from its path (/2s/·_2s → 2s, /6s/·_6s → 6s, else LL).

server_build

• go-live: main.Version (set via -ldflags -X) → api.BuildVersion → X-Served-By becomes go-live/<build> (was static go-live). All 11 header sites now call servedBy().
• Dockerfile: go-live builds with -ldflags "-X main.Version=${VERSION}", reusing the same VERSION build-arg go-proxy already consumes.
• forwarder: the proxy already captures X-Served-By into network_requests.response_headers; find.go extracts it per play via a server_builds CTE (JSON header parse), and scenario.go strips the go-live/ prefix (empty for un-stamped dev builds).

Plumbed through

forwarder.yaml Scenario (+manifest_variant enum, +server_build) → regenerated harness-cli client → v2-repo.ts Scenario → Sessions.vue scenario chips (variant, build).

Test

• Table tests for manifestVariant + serverBuild; server-side-fields enrich test.
• go build/vet/test (forwarder, harness-cli, go-live), vue-tsc, vite build all clean.
• ldflags injection verified end-to-end (binary prints build "v2.0.0-test" → header go-live/v2.0.0-test).
• find.go Sprintf verb/arg count re-balanced (17 = 17) after adding the CTE.

Rollout

• Forwarder rebuild (make analytics-rebuild-forwarder) lights up manifest_variant immediately — derived from existing rows.
• server_build needs a container rebuild (so go-live starts emitting the stamped header) and then populates for plays served afterward; older plays show no build (correct — that build info wasn't captured at the time).

Closes #679

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