feat(pi5): RP1 backend UI, gpio slowdown guidance, and hardware init error banner (#337)

* feat(pi5): expose RP1 backend selector, fix gpio defaults, surface init failures in web UI

- Add rp1_rio select (PIO/RIO) to Display Settings hardware config section;
  saved via /api/v3/config/main with 0-or-1 validation — previously the key
  existed in config.json but was not editable from the UI
- Update gpio_slowdown help text with per-model guidance (Pi 3: 3, Pi 4: 4,
  Pi 5: 4–5) and raise max from 5 → 10 to match full library range
- Fix gpio_slowdown Python fallback default from 2 → 3 (only affects edge case
  where the runtime config section is absent; explicit config values are unchanged)
- display_manager writes /tmp/led_matrix_hw_status.json at startup: ok/error;
  Display Settings page fetches it and shows a yellow warning banner when the
  matrix failed to initialize, including Pi 5 remediation steps
- Add GET /api/v3/hardware/status endpoint that reads the status file
- Improve fallback error log to include Pi 5 rebuild hint

Pi 3/4 users: rp1_rio=0 is set in config but silently ignored by the library
on non-RP1 hardware; all other changes are additive or tighten defaults only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(pi5): correct gpio_slowdown guidance — Pi 5 PIO default is 1, not 4-5

The upstream library defaults gpio_slowdown to 1 for Pi 5 (IsPi4() ? 2 : 1).
In PIO mode the value is a pixel-clock divisor, so 4-5 was unnecessarily
conservative advice. Updated help text and error log to reflect the actual
range (1-3 typical for Pi 5 PIO; inverted effect in RIO mode).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(security): atomic hw-status write, narrow bare excepts, urllib3 CVE floor

- display_manager: replace open()+bare-except with tempfile.mkstemp→fsync→
  chmod(0o600)→os.replace; adds symlink guard and logs errors via logger
  instead of swallowing them silently; pull json/tempfile to module imports
- display_manager cleanup(): narrow broad `except Exception: pass` to
  (OSError, RuntimeError, ValueError, MemoryError) with debug log
- api_v3 get_hardware_status(): catch json.JSONDecodeError and PermissionError
  explicitly; log full traceback server-side; return generic "Unable to read
  hardware status" to client instead of leaking str(e)
- march-madness/requirements.txt: bump urllib3 floor 2.2.2→2.6.3 (CVE fix)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(template): apply |int filter to rp1_rio comparisons in display.html

Without |int, a string-typed value (e.g. from a hand-edited config.json)
causes both selected tests to fail and the select renders with no option
pre-selected. Matches the existing pattern used for multiplexing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/blueprints/api_v3.py

@@ -699,7 +699,7 @@ def save_main_config():
 
         # Handle display settings
         display_fields = ['rows', 'cols', 'chain_length', 'parallel', 'brightness', 'hardware_mapping',
-                         'gpio_slowdown', 'scan_mode', 'disable_hardware_pulsing', 'inverse_colors', 'show_refresh_rate',
+                         'gpio_slowdown', 'rp1_rio', 'scan_mode', 'disable_hardware_pulsing', 'inverse_colors', 'show_refresh_rate',
                          'pwm_bits', 'pwm_dither_bits', 'pwm_lsb_nanoseconds', 'limit_refresh_rate_hz', 'use_short_date_format',
                          'max_dynamic_duration_seconds', 'led_rgb_sequence', 'multiplexing', 'panel_type']
 
@@ -747,6 +747,14 @@ def save_main_config():
             # Handle runtime settings
             if 'gpio_slowdown' in data:
                 current_config['display']['runtime']['gpio_slowdown'] = int(data['gpio_slowdown'])
+            if 'rp1_rio' in data:
+                try:
+                    rp1_val = int(data['rp1_rio'])
+                    if rp1_val not in (0, 1):
+                        return jsonify({'status': 'error', 'message': "rp1_rio must be 0 (PIO) or 1 (RIO)"}), 400
+                    current_config['display']['runtime']['rp1_rio'] = rp1_val
+                except (ValueError, TypeError):
+                    return jsonify({'status': 'error', 'message': "rp1_rio must be 0 or 1"}), 400
 
             # Handle checkboxes - coerce to bool to ensure proper JSON types
             for checkbox in ['disable_hardware_pulsing', 'inverse_colors', 'show_refresh_rate']:
@@ -1527,6 +1535,23 @@ def execute_system_action():
         print(error_details)
         return jsonify({'status': 'error', 'message': str(e), 'details': error_details}), 500
 
+@api_v3.route('/hardware/status', methods=['GET'])
+def get_hardware_status():
+    """Return LED matrix hardware initialization status written by display_manager at startup."""
+    status_path = "/tmp/led_matrix_hw_status.json"  # nosec B108
+    try:
+        with open(status_path) as f:
+            hw_data = json.load(f)
+        return jsonify({"status": "success", "data": hw_data})
+    except FileNotFoundError:
+        return jsonify({"status": "success", "data": {"ok": None, "error": "Display service not yet started"}})
+    except (json.JSONDecodeError, PermissionError):
+        logger.error("Failed to read hardware status file", exc_info=True)
+        return jsonify({"status": "error", "message": "Unable to read hardware status"}), 500
+    except Exception:
+        logger.error("Unexpected error reading hardware status", exc_info=True)
+        return jsonify({"status": "error", "message": "Unable to read hardware status"}), 500
+
 @api_v3.route('/display/current', methods=['GET'])
 def get_display_current():
     """Get current display state"""