shared: auto-emit Langfuse trace+span per HTTP request — closes OPEN #2

Adds langfuseMiddleware in internal/shared so every daemon's
shared.Run gets free production-traffic trace visibility when
LANGFUSE_URL + LANGFUSE_PUBLIC_KEY + LANGFUSE_SECRET_KEY are set.
Same env names + file shape as the multi_coord_stress driver, so
operators ship one /etc/lakehouse/langfuse.env across the deploy.

Wiring is auth-gated: middleware runs INSIDE the RequireAuth group,
so 401s from credential-stuffing don't pollute traces. /health is
exempt so LB probes don't either. Missing env vars → nil client →
middleware is a passthrough no-op (fail-open per ADR-005 5.1).

Bundled deploy:
- langfuse.env.example template (mode 0640, root:lakehouse)
- 11 systemd units gain `EnvironmentFile=-/etc/lakehouse/langfuse.env`
  (leading - so missing file = OK)
- REPLICATION.md bootstrap section documents setup

Tests (4): nil passthrough, /health bypass, real-request emission,
status-writer wrapping. All green.

STATE_OF_PLAY OPEN list: 5 rows → 4 rows.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/etc-lakehouse/langfuse.env.example

@@ -0,0 +1,19 @@
+# /etc/lakehouse/langfuse.env — Langfuse trace observability.
+#
+# Mode 0640, root:lakehouse. Loaded by every daemon's systemd unit
+# via EnvironmentFile=-/etc/lakehouse/langfuse.env. The `-` prefix
+# means "missing file is OK" — Langfuse is optional. If the file is
+# missing or any of the three env vars below is empty, every daemon
+# runs without Langfuse and with no logged warnings (best-effort).
+#
+# When all three are set, every authenticated HTTP request to every
+# daemon emits one Langfuse trace + span, browseable at LANGFUSE_URL.
+# /health is exempt to keep LB probes from polluting traces.
+#
+# Same env vars + same shape as the file the multi_coord_stress
+# driver reads — operators ship one langfuse.env across the whole
+# deploy.
+
+LANGFUSE_URL=http://localhost:3001
+LANGFUSE_PUBLIC_KEY=pk-lf-staffing
+LANGFUSE_SECRET_KEY=sk-lf-staffing-secret