Wave 147 Phase 2: Fix .env loading in integration tests

## Problem
Integration tests failed to load JWT_SECRET from .env file, causing 19/49 E2E tests to fail with authentication errors.

## Root Cause
cargo test doesn't automatically load .env files. Tests need explicit dotenvy integration.

## Solution
1. Added dotenvy dependency to integration_tests/Cargo.toml
2. Added automatic .env loading to get_test_jwt_secret() function
3. Made .env loading idempotent (safe to call multiple times)

## Test Results
- Service Health: 26/26 passing (100%)
- Backtesting: 23/23 passing (100%)
- Total: 49/49 passing (100%)

## Files Modified
- services/integration_tests/Cargo.toml (+3 lines)
- services/integration_tests/tests/common/auth_helpers.rs (+3 lines)

## Agents
- Agent 401: .env loading fix
- Agent 402: Final E2E validation (100%)
- Agent 403: Git commit

🎉 Generated with Claude Code

services/integration_tests/Cargo.toml

@@ -38,6 +38,9 @@ uuid = { workspace = true, features = ["v4"] }
 # Time handling
 chrono = { workspace = true }
 
+# Environment variables
+dotenvy = "0.15"
+
 [dev-dependencies]
 # Test utilities
 serial_test.workspace = true

services/integration_tests/tests/common/auth_helpers.rs

@@ -200,6 +200,9 @@ impl TestAuthConfig {
 /// 2. Set JWT_SECRET in .env file
 /// 3. Load .env before running tests (e.g., `source .env` or use dotenv)
 pub fn get_test_jwt_secret() -> String {
+    // Load .env file for JWT_SECRET (idempotent, safe to call multiple times)
+    let _ = dotenvy::dotenv();
+
     std::env::var("JWT_SECRET").expect(
         "FATAL: JWT_SECRET must be set in .env file for E2E tests\n\
          \n\