perf(db): batch retention deletes and tune autovacuum

[MrAlders0n]

Batch the packet and route retention deletes, tune autovacuum on the high-churn tables, and drop the default packets.retention to 7 days.

Upgrade notes:

• If packets.retention isn't set, packet and chat history now keeps 7 days instead of 30.
• One-time cleanup of existing index bloat: REINDEX TABLE CONCURRENTLY known_routes;

Sizing check for a follow-up on resolved_endpoints:

SELECT count(*), avg(pg_column_size(resolved_endpoints)), max(pg_column_size(resolved_endpoints))
FROM packet_observations TABLESAMPLE SYSTEM (0.1);