fix: use absolute paths for database to ensure persistence

Critical bug fix: Path(__file__).parent.parent returns a relative path,
causing the database to be created in different locations depending on
the working directory. This caused data loss on container restarts.

Changes:
- Add .resolve() to BASE_DIR in src/config.py and migrations/env.py
- Fix admin dashboard showing 0 for participant count (was hardcoded)
- Fix exchange detail page to show actual participant list
- Add startup diagnostics to entrypoint.sh for troubleshooting

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/config.py

@@ -12,8 +12,8 @@ from pathlib import Path
 class Config:
     """Base configuration class with common settings."""
 
-    # Base paths
-    BASE_DIR = Path(__file__).parent.parent
+    # Base paths - use resolve() to ensure absolute paths
+    BASE_DIR = Path(__file__).parent.parent.resolve()
     DATA_DIR = BASE_DIR / "data"
 
     # Security