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.

2 files changed, 73 insertions, 9 deletions

diff --git a/src/config.rs b/src/config.rs
index b4dd853..f04b7d8 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -2,6 +2,40 @@ use anyhow::{Context, Result};
 use serde::{Deserialize, Serialize};
 use std::env;
 
+/// Database backend type for the relay
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+pub enum DatabaseBackend {
+    /// In-memory database (default, fastest, no persistence)
+    Memory,
+    /// NostrDB backend (persistent, optimized for Nostr)
+    NostrDb,
+    /// LMDB backend (persistent, general purpose)
+    Lmdb,
+}
+
+impl Default for DatabaseBackend {
+    fn default() -> Self {
+        Self::Memory
+    }
+}
+
+impl std::str::FromStr for DatabaseBackend {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        match s.to_lowercase().as_str() {
+            "memory" => Ok(Self::Memory),
+            "nostrdb" => Ok(Self::NostrDb),
+            "lmdb" => Ok(Self::Lmdb),
+            _ => Err(anyhow::anyhow!(
+                "Invalid database backend: {}. Valid options: memory, nostrdb, lmdb",
+                s
+            )),
+        }
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Config {
     pub domain: String,
@@ -11,6 +45,7 @@ pub struct Config {
     pub git_data_path: String,
     pub relay_data_path: String,
     pub bind_address: String,
+    pub database_backend: DatabaseBackend,
 }
 
 impl Config {
@@ -18,6 +53,12 @@ impl Config {
         // Load .env file if present
         dotenvy::dotenv().ok();
 
+        // Parse database backend from environment
+        let database_backend = env::var("NGIT_DATABASE_BACKEND")
+            .ok()
+            .and_then(|s| s.parse().ok())
+            .unwrap_or_default();
+
         Ok(Config {
             domain: env::var("NGIT_DOMAIN").unwrap_or_else(|_| "localhost:8080".to_string()),
             owner_npub: env::var("NGIT_OWNER_NPUB").context("NGIT_OWNER_NPUB must be set")?,
@@ -31,6 +72,7 @@ impl Config {
                 .unwrap_or_else(|_| "./data/relay".to_string()),
             bind_address: env::var("NGIT_BIND_ADDRESS")
                 .unwrap_or_else(|_| "127.0.0.1:8080".to_string()),
+            database_backend,
         })
     }
 }
diff --git a/src/nostr/builder.rs b/src/nostr/builder.rs
index 09b2fe2..547db8e 100644
--- a/src/nostr/builder.rs
+++ b/src/nostr/builder.rs
@@ -11,7 +11,7 @@ use nostr::prelude::{Alphabet, SingleLetterTag};
 use nostr::{EventId, Filter, Kind, PublicKey};
 use nostr_relay_builder::prelude::*;
 
-use crate::config::Config;
+use crate::config::{Config, DatabaseBackend};
 use crate::nostr::events::{
     validate_announcement, validate_state, KIND_REPOSITORY_ANNOUNCEMENT, KIND_REPOSITORY_STATE,
 };
@@ -397,14 +397,36 @@ pub fn create_relay(config: &Config) -> Result<LocalRelay> {
     // Determine database path
     let db_path = Path::new(&config.relay_data_path);
 
-    // Create database - using in-memory for now, can switch to persistent later
-    // TODO: Add configuration for NostrDB or LMDB backends
-    let database = Arc::new(MemoryDatabase::with_opts(MemoryDatabaseOptions {
-        events: true,
-        max_events: Some(100_000),
-    }));
-
-    tracing::info!("Using in-memory database (path: {})", db_path.display());
+    // Create database based on configuration
+    let database = match config.database_backend {
+        DatabaseBackend::Memory => {
+            tracing::info!("Using in-memory database (no persistence)");
+            Arc::new(MemoryDatabase::with_opts(MemoryDatabaseOptions {
+                events: true,
+                max_events: Some(100_000),
+            }))
+        }
+        DatabaseBackend::NostrDb => {
+            tracing::info!("Using NostrDB backend at: {}", db_path.display());
+            // TODO: Implement NostrDB backend once nostr-relay-builder supports it
+            // For now, fall back to memory database
+            tracing::warn!("NostrDB backend not yet implemented, using in-memory database");
+            Arc::new(MemoryDatabase::with_opts(MemoryDatabaseOptions {
+                events: true,
+                max_events: Some(100_000),
+            }))
+        }
+        DatabaseBackend::Lmdb => {
+            tracing::info!("Using LMDB backend at: {}", db_path.display());
+            // TODO: Implement LMDB backend once nostr-relay-builder supports it
+            // For now, fall back to memory database
+            tracing::warn!("LMDB backend not yet implemented, using in-memory database");
+            Arc::new(MemoryDatabase::with_opts(MemoryDatabaseOptions {
+                events: true,
+                max_events: Some(100_000),
+            }))
+        }
+    };
 
     // Build relay with GRASP-01 validation
     // Clone Arc for the write policy so both relay and policy can access the database