retention: opt-in pruning for inactive_nodes, node_changes, ping_triggers and soft-deleted observers #329

Some tables are never pruned today, so an operator cannot honour a fixed retention period for node and observer data:

Proposal

• Add opt-in retention knobs in retention, for example inactiveNodeDays, nodeChangeDays, pingTriggerDays and observerPurgeDays. When unset, behaviour stays as today (upstream-compatible).
• Run the deletes in the ingestor's existing daily prune and at startup, in batches inside WriterTx, like pruneBatches.
• ping_triggers should follow the transmission prune (delete triggers whose transmission is gone, or apply its own age).
• Log the counts, the same way the existing [prune] lines do.

Acceptance

• For each knob, a test that is red on master and green after: old rows are removed, recent rows are kept, and an unset knob changes nothing.
• No regression in the Ping Scores history: entries already marked data_pruned must still render.
• Every write stays in cmd/ingestor.
• Document the knobs in config.example.json.