feat: add database backend configuration options

Add environment variable configuration for database backend selection:
- Added DatabaseBackend enum (memory, nostrdb, lmdb) in src/config.rs
- Updated relay builder to use configured backend in src/nostr/builder.rs
- Added NGIT_DATABASE_BACKEND to .env.example with documentation
- Updated docs/reference/configuration.md with backend comparison table

NostrDB and LMDB backends prepared for future implementation when
nostr-relay-builder adds support. Currently defaults to in-memory
database with warning logs when persistent backends are selected.

1 files changed, 41 insertions, 0 deletions

diff --git a/docs/reference/configuration.md b/docs/reference/configuration.md
index fc7bbe0..e2ec9aa 100644
--- a/docs/reference/configuration.md
+++ b/docs/reference/configuration.md
@@ -224,6 +224,47 @@ NGIT_RELAY_DATA_PATH=/mnt/ssd/relay-data
 
 ---
 
+#### `NGIT_DATABASE_BACKEND`
+
+**Description:** Database backend type for storing Nostr events
+**Type:** String (enum: memory, nostrdb, lmdb)
+**Default:** `memory`
+**Required:** No
+
+**Valid Values:**
+- `memory` - In-memory database (default, fastest, no persistence)
+- `nostrdb` - NostrDB backend (persistent, optimized for Nostr) [Not yet implemented]
+- `lmdb` - LMDB backend (persistent, general purpose) [Not yet implemented]
+
+**Examples:**
+```bash
+# Development (default, no persistence)
+NGIT_DATABASE_BACKEND=memory
+
+# Production with NostrDB (when implemented)
+NGIT_DATABASE_BACKEND=nostrdb
+
+# Production with LMDB (when implemented)
+NGIT_DATABASE_BACKEND=lmdb
+```
+
+**Comparison:**
+
+| Backend | Persistence | Performance | Use Case |
+|---------|-------------|-------------|----------|
+| memory | No | Fastest | Development, testing |
+| nostrdb | Yes | High | Production (Nostr-optimized) |
+| lmdb | Yes | High | Production (general purpose) |
+
+**Notes:**
+- `memory` backend loses all data on restart
+- NostrDB and LMDB backends will use `NGIT_RELAY_DATA_PATH` for storage
+- NostrDB and LMDB are planned features, not yet available
+- Default `memory` backend suitable for development and testing only
+- Production deployments should use persistent backends when available
+
+---
+
 ### Logging Configuration
 
 #### `RUST_LOG`