Add doc for workers architecture

assets/documentation/Workers processing/04-Database-Schema.md

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+# Database Schema & Queue Structure
+
+This diagram shows the entity relationships between BundleBacktestRequest, BacktestJob, and User entities.
+
+```mermaid
+erDiagram
+    BundleBacktestRequest ||--o{ BacktestJob : "has many"
+    BacktestJob }o--|| User : "belongs to"
+    
+    BundleBacktestRequest {
+        UUID RequestId PK
+        INT UserId FK
+        STRING Status
+        INT TotalBacktests
+        INT CompletedBacktests
+        INT FailedBacktests
+        DATETIME CreatedAt
+        DATETIME CompletedAt
+        STRING UniversalConfigJson
+        STRING DateTimeRangesJson
+        STRING MoneyManagementVariantsJson
+        STRING TickerVariantsJson
+    }
+    
+    BacktestJob {
+        UUID Id PK
+        UUID BundleRequestId FK
+        STRING JobType
+        STRING Status
+        INT Priority
+        TEXT ConfigJson
+        TEXT CandlesJson
+        INT ProgressPercentage
+        INT CurrentBacktestIndex
+        INT TotalBacktests
+        INT CompletedBacktests
+        DATETIME CreatedAt
+        DATETIME StartedAt
+        DATETIME CompletedAt
+        TEXT ResultJson
+        TEXT ErrorMessage
+        STRING AssignedWorkerId
+        DATETIME LastHeartbeat
+    }
+    
+    User {
+        INT Id PK
+        STRING Name
+    }
+```
+
+## Table Descriptions
+
+### BundleBacktestRequest
+- Represents a bundle of multiple backtest jobs
+- Contains variant configurations (date ranges, money management, tickers)
+- Tracks overall progress across all jobs
+
+### BacktestJob
+- Individual backtest execution unit
+- Contains serialized config and candles
+- Tracks progress, worker assignment, and heartbeat
+- Links to parent bundle request
+
+### Key Indexes
+- `idx_status_priority`: For efficient job claiming (Status, Priority DESC, CreatedAt)
+- `idx_bundle_request`: For bundle progress queries
+- `idx_assigned_worker`: For worker health monitoring
+