fix(ingest): exact WAL backlog and org queue counters, no metering for refused batches - #2440

🟢 Confidence 10/10 · safe to merge. Backlog and org-byte arithmetic verified by reading the lane sync/delivery ordering; the Rust test job has not finished at this head.
_{no findings · tests covered · risk high}

Fixes five WAL/metering defects in the ingest gateway plus two encoder mismatches in the Go collector exporter, and drops the unused INGEST_TINYBIRD_CONCURRENCY_PER_SHARD. The counter and billing changes hold up; safe to merge.

• while_caller_waits subtracts a skipped frame's bytes from committed_bytes, so backlog returns to zero
• export_and_mark releases org queue bytes before marked?, covering a failed cursor write
• batch_was_stored skips metering and key use when a relay was refused and nothing was stored
• read_wal_frame grows the payload buffer instead of allocating the declared length up front

Before merge

• Infra · Check every environment has what this config now expects · apps/ingest/Dockerfile.prebuilt
• Infra · Check every environment has what this config now expects · apps/ingest/alchemy.run.ts
• Manual · Drop INGEST_TINYBIRD_CONCURRENCY_PER_SHARD from the prd and dev deploy configs; nothing reads it at this head

_{4434067 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}