diff --git a/backend/.env.example b/backend/.env.example
index 5035a28b..968d01a5 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -1,7 +1,22 @@
-# Database
+# ─── Server ───────────────────────────────────────────────────────────────────
+# Port the Express server listens on (default: 3001)
+PORT=3001
+
+# Application environment mode ('development', 'production', or 'test')
+NODE_ENV=development
+
+# Comma-separated list of allowed origins for CORS. In development, if unset,
+# defaults to http://localhost:3000
+CORS_ALLOWED_ORIGINS="https://app.flowfi.xyz,https://flowfi.xyz"
+
+# Optional base URL for Swagger OpenAPI documentation server (e.g. http://localhost:3001)
+API_BASE_URL=http://localhost:3001
+
+# ─── Database ─────────────────────────────────────────────────────────────────
+# Main PostgreSQL database connection URL
 DATABASE_URL="postgresql://user:password@localhost:5433/flowfi?schema=public"
 
-# PostgreSQL pool settings
+# PostgreSQL pool settings:
 # Maximum database connections per backend process (default: 10)
 PG_POOL_MAX=10
 # How long an idle connection stays open before being closed (milliseconds, default: 30000)
@@ -11,51 +26,58 @@ PG_CONNECTION_TIMEOUT_MS=5000
 # Maximum time a PostgreSQL statement may run before cancellation (milliseconds, default: 30000)
 PG_STATEMENT_TIMEOUT_MS=30000
 
-# Server
-PORT=3001
-NODE_ENV=development
-CORS_ALLOWED_ORIGINS="https://app.flowfi.xyz,https://flowfi.xyz"
-# Comma-separated list of allowed origins for CORS. In development, if unset,
-# defaults to http://localhost:3000
-
-# Stellar Network (Testnet/Mainnet)
+# ─── Stellar & Soroban ────────────────────────────────────────────────────────
+# Stellar network target ('testnet' or 'mainnet')
 STELLAR_NETWORK=testnet
-STELLAR_HORIZON_URL=https://horizon-testnet.stellar.org
-
-# Enable sandbox mode
-SANDBOX_MODE_ENABLED=true
 
-# Optional: Use a separate database for sandbox
-# If not set, it will use {DATABASE_URL}_sandbox
-SANDBOX_DATABASE_URL="postgresql://user:password@localhost:5433/flowfi_sandbox?schema=public"
+# Stellar Horizon REST API server endpoint
+STELLAR_HORIZON_URL=https://horizon-testnet.stellar.org
 
-# ─── Soroban Event Indexer ────────────────────────────────────────────────────
-# Soroban RPC endpoint (testnet default shown)
+# Soroban RPC endpoint URL (testnet default shown)
 SOROBAN_RPC_URL=https://soroban-testnet.stellar.org
 
-# Deployed FlowFi stream contract address (C...)
-# Leave empty to disable the event indexer
+# Deployed FlowFi stream contract address (C...). Leave empty to disable indexers
 STREAM_CONTRACT_ID=
 
-# How often the indexer polls for new events (milliseconds, default: 5000)
+# Server-side Stellar secret key (S...) used to sign and submit on-chain transactions
+# (cancel_stream, top_up_stream). Must be funded on the target network.
+KEEPER_SECRET_KEY=
+
+# Max stream creation requests per wallet per minute (default: 10)
+STREAM_CREATE_RATE_LIMIT=10
+
+# Timeout for Soroban RPC requests in milliseconds (default: 10000)
+SOROBAN_RPC_TIMEOUT_MS=10000
+
+# Maximum retry attempts for failed Soroban RPC requests (default: 2)
+SOROBAN_RPC_MAX_RETRIES=2
+
+# Base delay in milliseconds for exponential backoff during Soroban RPC retries (default: 250)
+SOROBAN_RPC_RETRY_BASE_MS=250
+
+# ─── Soroban Event Indexer ────────────────────────────────────────────────────
+# Polling interval in milliseconds for the background Soroban event worker (default: 5000)
 INDEXER_POLL_INTERVAL_MS=5000
 
-# Ledger sequence to start indexing from on first run (0 = latest)
+# Ledger sequence for event worker to start indexing from on first run (0 = latest, default: 0)
 INDEXER_START_LEDGER=0
 
-# Max stream creation requests per wallet per minute (default: 10)
-STREAM_CREATE_RATE_LIMIT=10
+# Polling interval in milliseconds for Soroban indexer service (default: 15000)
+SOROBAN_INDEXER_POLL_MS=15000
 
-# Server-side Stellar secret key used to sign and submit on-chain transactions
-# (cancel_stream, top_up_stream). Must be funded on the target network.
-KEEPER_SECRET_KEY=
+# Ledger sequence to start indexing from for Soroban indexer service (default: 0)
+SOROBAN_INDEXER_START_LEDGER=0
 
 # ─── Auth ─────────────────────────────────────────────────────────────────────
-# Secret used to sign JWTs (generate with: openssl rand -hex 32)
+# Secret used to sign JWTs (generate with: openssl rand -hex 32). Required in production.
 JWT_SECRET=
 
-# Stellar public key of the admin user (for /v1/admin/* endpoints)
+# Stellar public key (G...) of the admin user (for /v1/admin/* endpoints)
 ADMIN_PUBLIC_KEY=
+
+# Interval in milliseconds to sweep and prune expired auth challenges (default: 300000)
+AUTH_CHALLENGE_SWEEP_INTERVAL_MS=300000
+
 # ─── Redis (optional) ────────────────────────────────────────────────────────
 # When set, enables horizontal SSE scaling via Redis pub/sub so events
 # emitted by any backend instance are broadcast to clients on all instances.
@@ -63,8 +85,36 @@ ADMIN_PUBLIC_KEY=
 REDIS_URL=
 
 # ─── Caching (optional) ───────────────────────────────────────────────────────
-# Time in milliseconds between periodic sweeps to prune expired memory cache
-# entries (default: 60000)
+# Time in milliseconds between periodic sweeps to prune expired memory cache entries (default: 60000)
 MEMORY_CACHE_SWEEP_MS=60000
 
+# Cache time-to-live (TTL) for claimable amount calculations in milliseconds (default: 5000)
+CLAIMABLE_CACHE_TTL_MS=5000
+
+# ─── Logging ──────────────────────────────────────────────────────────────────
+# Log output verbosity level ('error', 'warn', 'info', 'debug', default: info)
+LOG_LEVEL=info
+
+# ─── Streaming & Server-Sent Events (SSE) ─────────────────────────────────────
+# Maximum concurrent SSE client connections allowed (default: 10000)
+MAX_SSE_CONNECTIONS=10000
+
+# ─── Sandbox Mode ─────────────────────────────────────────────────────────────
+# Enable or disable sandbox mode ('true' or 'false', default: true)
+SANDBOX_MODE_ENABLED=true
+
+# Optional: Separate PostgreSQL connection URL for sandbox mode.
+# If not set, defaults to {DATABASE_URL}_sandbox
+SANDBOX_DATABASE_URL="postgresql://user:password@localhost:5433/flowfi_sandbox?schema=public"
+
+# Whether sandbox mode can be triggered via request headers ('true' or 'false', default: true)
+SANDBOX_ALLOW_HEADER=true
+
+# Whether sandbox mode can be triggered via query parameters ('true' or 'false', default: true)
+SANDBOX_ALLOW_QUERY_PARAM=true
+
+# Header name used to trigger sandbox mode (default: X-Sandbox-Mode)
+SANDBOX_HEADER_NAME=X-Sandbox-Mode
 
+# Query parameter name used to trigger sandbox mode (default: sandbox)
+SANDBOX_QUERY_PARAM_NAME=sandbox