Observability: Langfuse tracing + Postgres turn ledger + Grafana dashboards

[mr-j90]

Parent

Closes #8 — SPEC §8

What this delivers

Every turn dual-writes: a Langfuse trace (self-hosted v3 via the compose observability profile — 6 pinned services, headless-init dev keys, trace id pinned to turn_id via an enclosing observation) and a flat Postgres row (model, tools, latency, tokens, cost, error). Feedback dual-writes by turn_id: row update + Langfuse score (flushed), replacing the JSONL sink. Grafana provisions a 7-panel dashboard as code over the turns table (volume, latency p50/p95, route split, feedback rate, cost/day, turn-error rate, tokens/day), anonymous read.

Verified live: traced turn → Postgres row with real tokens/cost; trace id == turn_id (8 observations); feedback → row + score on that same trace; dashboard served without login; row lands via BackgroundTask even on client disconnect.

Review notes (two-axis /code-review; both axes converged on three hard findings — all fixed)

• UI feedback was silently landing nowhere: hooks refactor orphaned the turn_id patch; done handler now stores it (one line) and the API refuses to no-op silently (logs + skips the score when the row doesn't match, preventing store divergence).
• Grafana datasource password never resolved (${VAR:-default} isn't Grafana syntax) — plain env ref now.
• Client disconnects dropped the turn row — the write moved to a BackgroundTask, which Starlette runs even on disconnect; abandoned turns record error='client disconnected'.
• Also: Opus pricing corrected 3×→ $5/$25 per MTok; deterministic price matching + loud unknown-model fallback; 'Tool-error rate' panel renamed 'Turn-error rate' (it measures turn errors — honest label until tool-level errors are worth plumbing).
• Accepted/noted: hand-written pinned Langfuse stack instead of upstream's compose include (deliberate: one file, profile-gated, pinned); Langfuse span CM across generator yields can log a cosmetic detach warning on disconnect; per-write Postgres connections (fine at capstone volume).