feat(settings): per-option save, live worker sync, audit log, and rollback

A) Per-option Save Button
- Add per-setting Save button in settings.html (visible only when value changed)
- Button calls POST /api/settings/{key} directly; existing bulk Save retained
- Add Audit Log link in settings page header

B) Immediate Worker Sync
- New app/utils/settings_sync.py with notify_settings_updated() (Redis version key)
  and register_settings_reload_signal() (Celery task_prerun handler)
- Register signal in celery_worker.py at startup
- All API write paths call notify_settings_updated() after successful saves

C) Audit Log
- Add SettingsAuditLog model (key, old_value, new_value, changed_by, changed_at, action)
- save_setting_to_db / delete_setting_from_db accept changed_by and write audit entries
- New get_audit_log() service function (masks sensitive values)
- New GET /api/settings/audit-log endpoint (admin-only)
- New GET /admin/settings/audit-log view + audit_log.html template
- Visible to all admins (per clarified requirement)

D) Config Rollback / History
- New get_setting_history() and rollback_setting() service functions
- New GET /api/settings/{key}/history endpoint
- New POST /api/settings/{key}/rollback/{history_id} endpoint
- Rollback buttons in audit_log.html with confirmation dialog
- Tests: 25 new tests covering audit log, rollback, worker sync helpers, and API endpoints

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/models.py

@@ -1,6 +1,7 @@
 # app/models.py
 
-from sqlalchemy import Boolean, Column, DateTime, ForeignKey, Integer, String, Text, UniqueConstraint, func
+from sqlalchemy import (Boolean, Column, DateTime, ForeignKey, Integer, String,
+                        Text, UniqueConstraint, func)
 
 from app.database import Base
 
@@ -69,21 +70,33 @@ class FileProcessingStep(Base):
 
     id = Column(Integer, primary_key=True, index=True)
     file_id = Column(Integer, ForeignKey(_FILES_ID_FK), nullable=False, index=True)
-    step_name = Column(String, nullable=False, index=True)  # e.g., "hash_file", "upload_to_dropbox"
-    status = Column(String, nullable=False)  # "pending", "in_progress", "success", "failure", "skipped"
+    step_name = Column(
+        String, nullable=False, index=True
+    )  # e.g., "hash_file", "upload_to_dropbox"
+    status = Column(
+        String, nullable=False
+    )  # "pending", "in_progress", "success", "failure", "skipped"
     started_at = Column(DateTime(timezone=True), nullable=True)  # When step started
-    completed_at = Column(DateTime(timezone=True), nullable=True)  # When step finished (success/failure)
+    completed_at = Column(
+        DateTime(timezone=True), nullable=True
+    )  # When step finished (success/failure)
     error_message = Column(Text, nullable=True)  # Error message if status is "failure"
     created_at = Column(DateTime(timezone=True), server_default=func.now())
-    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+    updated_at = Column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
 
-    __table_args__ = (UniqueConstraint("file_id", "step_name", name="unique_file_step"),)
+    __table_args__ = (
+        UniqueConstraint("file_id", "step_name", name="unique_file_step"),
+    )
 
 
 class ProcessingLog(Base):
     __tablename__ = "processing_logs"
     id = Column(Integer, primary_key=True, index=True)
-    file_id = Column(Integer, ForeignKey(_FILES_ID_FK), nullable=True)  # Optional file association
+    file_id = Column(
+        Integer, ForeignKey(_FILES_ID_FK), nullable=True
+    )  # Optional file association
     task_id = Column(String, index=True)  # Celery task ID
     step_name = Column(String)  # e.g., "OCR", "convert_to_pdf", "upload_s3"
     status = Column(String)  # "pending", "in_progress", "success", "failure"
@@ -98,7 +111,29 @@ class ApplicationSettings(Base):
     __tablename__ = "application_settings"
 
     id = Column(Integer, primary_key=True, index=True)
-    key = Column(String, unique=True, index=True, nullable=False)  # Setting key (e.g., 'database_url')
-    value = Column(String, nullable=True)  # Setting value (stored as string, converted as needed)
+    key = Column(
+        String, unique=True, index=True, nullable=False
+    )  # Setting key (e.g., 'database_url')
+    value = Column(
+        String, nullable=True
+    )  # Setting value (stored as string, converted as needed)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
-    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+    updated_at = Column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+
+class SettingsAuditLog(Base):
+    """Audit log for all configuration changes made via the settings UI."""
+
+    __tablename__ = "settings_audit_log"
+
+    id = Column(Integer, primary_key=True, index=True)
+    key = Column(String, nullable=False, index=True)  # Setting key that was changed
+    old_value = Column(String, nullable=True)  # Previous value (None if first-time set)
+    new_value = Column(String, nullable=True)  # New value (None if deleted)
+    changed_by = Column(
+        String, nullable=False
+    )  # Username of the admin who made the change
+    changed_at = Column(DateTime(timezone=True), server_default=func.now(), index=True)
+    action = Column(String, nullable=False)  # "update" or "delete"