Add defensive relay features with rate limiting and connection limits

Implement defensive measures to protect against DoS attacks:
- Add explicit rate limits (500 subscriptions, 60 events/min per connection)
- Add total connection limit (default: 500, configurable via NGIT_MAX_CONNECTIONS)
- Update configuration across all 4 locations (src, nix, docs, .env.example)

Per-IP rate limiting deferred until abuse is detected in production or
implemented in rust-nostr relay-builder to benefit the entire Nostr ecosystem.

Documentation added explaining the defensive features and rationale.
Detailed analysis of other relay implementations preserved in commit history.

1 files changed, 42 insertions, 0 deletions

diff --git a/docs/explanation/README.md b/docs/explanation/README.md
index f477b73..58cc46f 100644
--- a/docs/explanation/README.md
+++ b/docs/explanation/README.md
@@ -151,6 +151,48 @@ Explanation documentation helps you **understand concepts** and design decisions
 
 ---
 
+### [Defensive Measures & Rate Limiting](defensive-measures.md)
+**Protection against abuse, spam, and denial-of-service attacks**
+
+**Topics:**
+- Connection and subscription management
+- Event publishing rate limits
+- Content filtering (blacklists/whitelists)
+- Event validation plugin system (WritePolicy/QueryPolicy)
+- Relay health management (naughty list, exponential backoff)
+- Privacy-preserving IP tracking
+- Future enhancements (per-IP rate limiting)
+
+**Read when:** You want to understand how ngit-grasp protects against abuse and what defensive features are available
+
+---
+
+### [GRASP-05 Archive Mode](grasp-05-archive.md)
+**Read-only mirroring of repositories**
+
+**Topics:**
+- Archive whitelist configuration
+- Archive-all mode
+- Read-only mode defaults
+- Use cases for backup/mirror relays
+
+**Read when:** You want to understand how to run an archive/backup relay
+
+---
+
+### [Deletion Requests](deletion-requests.md)
+**Handling repository and event deletion**
+
+**Topics:**
+- Deletion request architecture
+- Delete disrespector concept
+- Preventing left-pad scenarios
+- Archival policies
+
+**Read when:** You want to understand how ngit-grasp handles deletion events (planned feature)
+
+---
+
 ## Planned Explanation Documentation
 
 ### GRASP Protocol Design