fix: critical memory leak from per-session database connections (#554)

* fix: critical memory leak from per-session database connections (#542)

Each MCP session was creating its own database connection (~900MB),
causing OOM kills every ~20 minutes with 3-4 concurrent sessions.

Changes:
- Add SharedDatabase singleton pattern - all sessions share ONE connection
- Reduce session timeout from 30 min to 5 min (configurable)
- Add eager cleanup for reconnecting instances
- Fix telemetry event listener leak

Memory impact: ~900MB/session → ~68MB shared + ~5MB/session overhead

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Conceived by Romuald Czlonkowski - https://www.aiadvisors.pl/en

* fix: resolve test failures from shared database race conditions

- Fix `shutdown()` to respect shared database pattern (was directly closing)
- Add `await this.initialized` in both `close()` and `shutdown()` to prevent
  race condition where cleanup runs while initialization is in progress
- Add double-shutdown protection with `isShutdown` flag
- Export `SharedDatabaseState` type for proper typing
- Include error details in debug logs
- Add MCP server close to `shutdown()` for consistency with `close()`
- Null out `earlyLogger` in `shutdown()` for consistency

The CI test failure "The database connection is not open" was caused by:
1. `shutdown()` directly calling `this.db.close()` which closed the SHARED
   database connection, breaking subsequent tests
2. Race condition where `shutdown()` ran before initialization completed

Conceived by Romuald Członkowski - www.aiadvisors.pl/en

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* test: add unit tests for shared-database module

Add comprehensive unit tests covering:
- getSharedDatabase: initialization, reuse, different path error, concurrent requests
- releaseSharedDatabase: refCount decrement, double-release guard
- closeSharedDatabase: state clearing, error handling, re-initialization
- Helper functions: isSharedDatabaseInitialized, getSharedDatabaseRefCount

21 tests covering the singleton database connection pattern used to prevent
~900MB memory leaks per session.

Conceived by Romuald Członkowski - www.aiadvisors.pl/en

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/telemetry/batch-processor.ts

@@ -58,6 +58,13 @@ export class TelemetryBatchProcessor {
   private flushTimes: number[] = [];
   private deadLetterQueue: (TelemetryEvent | WorkflowTelemetry | WorkflowMutationRecord)[] = [];
   private readonly maxDeadLetterSize = 100;
+  // Track event listeners for proper cleanup to prevent memory leaks
+  private eventListeners: {
+    beforeExit?: () => void;
+    sigint?: () => void;
+    sigterm?: () => void;
+  } = {};
+  private started: boolean = false;
 
   constructor(
     private supabase: SupabaseClient | null,
@@ -72,6 +79,12 @@ export class TelemetryBatchProcessor {
   start(): void {
     if (!this.isEnabled() || !this.supabase) return;
 
+    // Guard against multiple starts (prevents event listener accumulation)
+    if (this.started) {
+      logger.debug('Telemetry batch processor already started, skipping');
+      return;
+    }
+
     // Set up periodic flushing
     this.flushTimer = setInterval(() => {
       this.flush();
@@ -83,17 +96,22 @@ export class TelemetryBatchProcessor {
       this.flushTimer.unref();
     }
 
-    // Set up process exit handlers
-    process.on('beforeExit', () => this.flush());
-    process.on('SIGINT', () => {
+    // Set up process exit handlers with stored references for cleanup
+    this.eventListeners.beforeExit = () => this.flush();
+    this.eventListeners.sigint = () => {
       this.flush();
       process.exit(0);
-    });
-    process.on('SIGTERM', () => {
+    };
+    this.eventListeners.sigterm = () => {
       this.flush();
       process.exit(0);
-    });
+    };
 
+    process.on('beforeExit', this.eventListeners.beforeExit);
+    process.on('SIGINT', this.eventListeners.sigint);
+    process.on('SIGTERM', this.eventListeners.sigterm);
+
+    this.started = true;
     logger.debug('Telemetry batch processor started');
   }
 
@@ -105,6 +123,20 @@ export class TelemetryBatchProcessor {
       clearInterval(this.flushTimer);
       this.flushTimer = undefined;
     }
+
+    // Remove event listeners to prevent memory leaks
+    if (this.eventListeners.beforeExit) {
+      process.removeListener('beforeExit', this.eventListeners.beforeExit);
+    }
+    if (this.eventListeners.sigint) {
+      process.removeListener('SIGINT', this.eventListeners.sigint);
+    }
+    if (this.eventListeners.sigterm) {
+      process.removeListener('SIGTERM', this.eventListeners.sigterm);
+    }
+    this.eventListeners = {};
+    this.started = false;
+
     logger.debug('Telemetry batch processor stopped');
   }