Simplex-style ephemeral event storage (configurable TTL)

[kwsantiago]

Summary

Implement Simplex-inspired ephemeral storage model where events are automatically deleted after a configurable retention period. This makes relays cheaper to run and supports use cases where permanent storage isn't needed.

Motivation

• Lower storage costs for relay operators
• Support "inbox-style" relays that only hold recent events
• Clients already manage local storage, so ephemeral relays can coexist with archival relays
• Aligns with Nostr's flexibility - users choose their relay mix

Proposed Features

1. Retention mode presets

Simple config option for relay operators to choose their storage model:

[retention]
mode = "ephemeral"  # archival | normal | ephemeral | custom

Mode	Behavior
archival	Store events indefinitely (current default)
normal	~1 year retention, profiles/relay lists longer
ephemeral	21-day retention (Simplex-style)
custom	Use per-kind settings below

2. Custom per-kind retention (when mode = "custom")

[retention]
mode = "custom"
default_days = 21
kind_0 = 90      # profiles
kind_1 = 21      # notes
kind_4 = 7       # DMs
kind_10002 = 90  # relay lists

3. Advertise retention in NIP-11 response

Add retention field to existing NIP-11 relay info so clients know the policy:

{
  "retention": [
    {"kinds": [0, 10002], "time": 7776000},
    {"kinds": [1, 4], "time": 1814400}
  ]
}

Implementation Notes

Existing infrastructure to build on:

• cleanupExpiredEvents() in store.zig - extend to check idx:created against TTL
• Background cleanup thread already runs hourly
• idx:created index enables efficient age-based lookups
• NIP-11 already implemented in nip11.zig - just add retention field

References

• Simplex SMP servers - 21-day message retention model
• NIP-40 (expiration tag) - already supported