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>

migrations/env.py

@@ -24,7 +24,8 @@ if config.config_file_name is not None:
 # Create minimal Flask app for migrations (without session initialization)
 # This avoids Flask-Session trying to create tables before migrations run
 app = Flask(__name__)
-BASE_DIR = Path(__file__).parent.parent
+# Use resolve() to ensure absolute paths - critical for database persistence
+BASE_DIR = Path(__file__).parent.parent.resolve()
 DATA_DIR = BASE_DIR / "data"
 DATA_DIR.mkdir(parents=True, exist_ok=True)