Refactor config loading and add status code docs

- Move config loading to dedicated ConfigLoader class with validation
- Add DATABASE_MIGRATIONS.md content to TROUBLESHOOTING.md
- Add API_STATUS_CODES.md documenting all API response codes
- Update runner.csx to use new config structure
- Add check_responses.py validation script
- Update config tests for new structure

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/TROUBLESHOOTING.md

@@ -331,6 +331,93 @@ sqlite3 /var/lib/bangui/bangui.db "PRAGMA integrity_check;"
 
 ---
 
+## Configuration Validation at Startup
+
+BanGUI validates configuration at startup. Errors raised here indicate misconfiguration that must be fixed before the application can start.
+
+### Database Parent Directory Does Not Exist
+
+**Symptom:** Application fails to start with: `Database parent directory does not exist: /path/to/parent`
+
+**Cause:** The parent directory of `BANGUI_DATABASE_PATH` does not exist.
+
+**Solution:**
+```bash
+mkdir -p /path/to/parent
+# Then restart BanGUI
+```
+
+---
+
+### Database Parent Directory Not Writable
+
+**Symptom:** Application fails to start with: `Database parent directory not writable: /path/to/parent`
+
+**Cause:** The process cannot write to the database parent directory.
+
+**Solution:**
+```bash
+chmod 755 /path/to/parent
+# Verify the user running BanGUI owns the directory or has write access
+```
+
+---
+
+### fail2ban Socket Not Readable
+
+**Symptom:** Application fails to start with: `fail2ban socket not readable: /path/to/socket`
+
+**Cause:** The socket file exists but is not readable by the BanGUI process.
+
+**Solution:**
+```bash
+chmod 644 /path/to/socket
+ls -la /path/to/socket
+```
+
+---
+
+### fail2ban Config Directory Does Not Exist
+
+**Symptom:** Application fails to start with: `fail2ban config directory does not exist: /path/to/config`
+
+**Cause:** `BANGUI_FAIL2BAN_CONFIG_DIR` points to a directory that does not exist.
+
+**Solution:**
+- Mount the fail2ban configuration directory at the expected path
+- Or adjust `BANGUI_FAIL2BAN_CONFIG_DIR` to point to the correct location
+- In Docker: add a volume mount for the fail2ban config directory
+
+---
+
+### GeoIP Database File Does Not Exist
+
+**Symptom:** Application fails to start with: `GeoIP database file does not exist: /path/to/GeoLite2-Country.mmdb`
+
+**Cause:** `BANGUI_GEOIP_DB_PATH` points to a file that does not exist.
+
+**Solution:**
+1. Download the MaxMind GeoLite2-Country database from https://dev.maxmind.com/geoip/geolite2-country
+2. Place it at the configured path, or update `BANGUI_GEOIP_DB_PATH` to the correct location
+3. Alternatively, set `BANGUI_GEOIP_DB_PATH` to `null` to disable GeoIP lookups
+
+---
+
+### session_secret Too Short or Weak
+
+**Symptom:** Application fails to start with: `session_secret must be at least 32 characters` or `session_secret is too weak`
+
+**Cause:** `BANGUI_SESSION_SECRET` is missing, too short, or contains common weak words.
+
+**Solution:**
+```bash
+# Generate a new secret
+python -c "import secrets; print(secrets.token_hex(32))"
+```
+Then set it in your `.env` file or environment variables.
+
+---
+
 ## Getting Help
 
 If issues persist after following this guide: