fix(api): surface DB query errors instead of 200 OK zeros

[Fl0p]

Summary

• Read handlers in internal/api/handler.go no longer discard Query/QueryRow errors; failures return JSON {"error":"..."} with a non-2xx status instead of a confident 200 of zeros.
• sql.ErrNoRows still yields empty/zero payloads (fresh install safe); every rows.Next() loop checks rows.Err().
• Table-driven tests cover /overview and /costs for query-failure → non-2xx and empty-DB → 200 with zeros.

Test plan

• go build ./...
• go test ./internal/api/...
• CI green on PR head
• Spot-check: forced query failure returns ErrorState-compatible JSON, empty DB still shows zeros

Made with Cursor

[coderabbitai]

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[Fl0p]

Merged to main as d3d51b0 via local squash (agent identity preserved per the merge policy), so GitHub will not auto-close this.

Rebased onto main first: feat(history) had since split handleHistory into historyRawSeries/historyUnionSeries, which have no ResponseWriter in scope — those helpers now return error and handleHistory maps it to queryFailed. That conflict was also why CI stopped scheduling: the PR was CONFLICTING, so GitHub could not build the merge ref.

One review fix on top: /sessions/{id} checked resp.SessionID == "" before err != nil, and Scan leaves the destination untouched on failure — so a broken DB reported 404 not found and the queryFailed branch was unreachable. Ordering fixed, regression test added.