feat(core): add database, models, queue and settings system

- Add SQLAlchemy database setup with session management
- Add Job model with status, priority and progress tracking
- Add QueueManager with priority queue and deduplication
- Add SystemSettings model for database-backed configuration
- Add SettingsService with caching and defaults
- Add SystemMonitor for CPU/RAM/GPU resource monitoring
- Add LanguageCode utilities (moved from root)

backend/core/settings_model.py

@@ -0,0 +1,74 @@
+"""Database model for system settings."""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, Text, DateTime
+from sqlalchemy.sql import func
+
+from backend.core.database import Base
+
+
+class SystemSettings(Base):
+    """
+    System settings stored in database.
+
+    Replaces .env configuration for dynamic settings management through WebUI.
+    Settings are organized by category and support different value types.
+    """
+
+    __tablename__ = "system_settings"
+
+    # Primary key
+    id = Column(Integer, primary_key=True, autoincrement=True)
+
+    # Setting identification
+    key = Column(String(255), nullable=False, unique=True, index=True)
+    value = Column(Text, nullable=True)  # Store as string, parse based on value_type
+
+    # Metadata
+    description = Column(Text, nullable=True)
+    category = Column(String(100), nullable=True, index=True)  # general, workers, transcription, scanner, bazarr
+    value_type = Column(String(50), nullable=True)  # string, integer, boolean, float, list
+
+    # Timestamps
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())
+
+    def __repr__(self):
+        """String representation."""
+        return f"<SystemSettings {self.key}={self.value}>"
+
+    def to_dict(self) -> dict:
+        """Convert to dictionary for API responses."""
+        return {
+            "id": self.id,
+            "key": self.key,
+            "value": self.value,
+            "description": self.description,
+            "category": self.category,
+            "value_type": self.value_type,
+            "created_at": self.created_at.isoformat() if self.created_at else None,
+            "updated_at": self.updated_at.isoformat() if self.updated_at else None,
+        }
+
+    def get_parsed_value(self):
+        """
+        Parse value based on value_type.
+
+        Returns:
+            Parsed value in appropriate Python type
+        """
+        if self.value is None:
+            return None
+
+        if self.value_type == "boolean":
+            return self.value.lower() in ("true", "1", "yes", "on")
+        elif self.value_type == "integer":
+            return int(self.value)
+        elif self.value_type == "float":
+            return float(self.value)
+        elif self.value_type == "list":
+            # Comma-separated values
+            return [v.strip() for v in self.value.split(",") if v.strip()]
+        else:  # string or unknown
+            return self.value
+
+