feat: ClickHouse over fetch, and OpenTelemetry spans for every statement - #33

🟢 Confidence 9/10 · safe to merge
Both the early-end kill guard and the peer attributes are pinned by tests the CI job actually ran; nothing new to fix.
_{quality 100/100 · no findings · tests covered · risk low · 1/1 new units observable}

Closes the KILL QUERY guard over body reads and early-stopped streams, names the peer on the client's spans, and keeps a URL path for proxy prefixes. Safe to merge.

• send returns its query_id; the JSON body read kills the query on interrupt
• executeStream kills unless the body was read to the end (Stream.onEnd/ensuring)
• Client spans carry server.address, server.port and peer.service, overridable via spanAttributes
• A path in url is kept; the config doc no longer claims it is refused

Fixed since the last review

• ✅ F1 · ClickHouse calls carry no peer.service, and their HTTP span is off

_{3cf5ef7 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}