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/database/database-adapter.ts

@@ -419,12 +419,36 @@ class BetterSQLiteStatement implements PreparedStatement {
 
 /**
  * Statement wrapper for sql.js
+ *
+ * IMPORTANT: sql.js requires explicit memory management via Statement.free().
+ * This wrapper automatically frees statement memory after each operation
+ * to prevent memory leaks during sustained traffic.
+ *
+ * See: https://sql.js.org/documentation/Statement.html
+ * "After calling db.prepare() you must manually free the assigned memory
+ *  by calling Statement.free()."
  */
 class SQLJSStatement implements PreparedStatement {
   private boundParams: any = null;
-  
+  private freed: boolean = false;
+
   constructor(private stmt: any, private onModify: () => void) {}
-  
+
+  /**
+   * Free the underlying sql.js statement memory.
+   * Safe to call multiple times - subsequent calls are no-ops.
+   */
+  private freeStatement(): void {
+    if (!this.freed && this.stmt) {
+      try {
+        this.stmt.free();
+        this.freed = true;
+      } catch (e) {
+        // Statement may already be freed or invalid - ignore
+      }
+    }
+  }
+
   run(...params: any[]): RunResult {
     try {
       if (params.length > 0) {
@@ -433,10 +457,10 @@ class SQLJSStatement implements PreparedStatement {
           this.stmt.bind(this.boundParams);
         }
       }
-      
+
       this.stmt.run();
       this.onModify();
-      
+
       // sql.js doesn't provide changes/lastInsertRowid easily
       return {
         changes: 1, // Assume success means 1 change
@@ -445,9 +469,12 @@ class SQLJSStatement implements PreparedStatement {
     } catch (error) {
       this.stmt.reset();
       throw error;
+    } finally {
+      // Free statement memory after write operation completes
+      this.freeStatement();
     }
   }
-  
+
   get(...params: any[]): any {
     try {
       if (params.length > 0) {
@@ -456,21 +483,24 @@ class SQLJSStatement implements PreparedStatement {
           this.stmt.bind(this.boundParams);
         }
       }
-      
+
       if (this.stmt.step()) {
         const result = this.stmt.getAsObject();
         this.stmt.reset();
         return this.convertIntegerColumns(result);
       }
-      
+
       this.stmt.reset();
       return undefined;
     } catch (error) {
       this.stmt.reset();
       throw error;
+    } finally {
+      // Free statement memory after read operation completes
+      this.freeStatement();
     }
   }
-  
+
   all(...params: any[]): any[] {
     try {
       if (params.length > 0) {
@@ -479,17 +509,20 @@ class SQLJSStatement implements PreparedStatement {
           this.stmt.bind(this.boundParams);
         }
       }
-      
+
       const results: any[] = [];
       while (this.stmt.step()) {
         results.push(this.convertIntegerColumns(this.stmt.getAsObject()));
       }
-      
+
       this.stmt.reset();
       return results;
     } catch (error) {
       this.stmt.reset();
       throw error;
+    } finally {
+      // Free statement memory after read operation completes
+      this.freeStatement();
     }
   }