docs: add bible/ as single source of architectural truth

- Created bible/ with hierarchical documentation (architecture, pricelists,
  vendor mapping, background tasks, data rules, patterns, API, operations, history)
- CLAUDE.md reduced to one instruction: read and follow the bible
- README.md reduced to quick start only
- Removed MEMORY.md and csv_export.md (content consolidated into bible/)
- Fixed stale facts found during audit: weighted_avg (not weighted_median),
  correct API route names (/export-csv, /recalculate-all)

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

bible/background-tasks.md

@@ -0,0 +1,86 @@
+# Background Tasks (Task Manager)
+
+## Principle
+
+**All long-running operations use the Task Manager. SSE (Server-Sent Events) must not be used.**
+
+---
+
+## Flow
+
+```
+1. Client sends POST request
+         │
+         ▼
+2. Handler creates task: taskManager.Submit(...)
+         │
+         ▼
+3. Handler returns { "task_id": "uuid" }
+         │
+         ▼
+4. Client polls GET /api/tasks/:id every 500ms
+         │
+         ▼
+5. On completed/error — show toast notification
+```
+
+---
+
+## Task Types
+
+| Constant | Purpose |
+|----------|---------|
+| `TaskTypeRecalculate` | Recalculate all component prices |
+| `TaskTypeStockImport` | Import stock file (.mxl/.xlsx) |
+| `TaskTypePricelistCreate` | Create pricelist (estimate/warehouse/competitor) |
+
+---
+
+## Task Structure
+
+```go
+type Task struct {
+    ID        string                 // Task UUID
+    Type      TaskType               // Task type
+    Status    TaskStatus             // running | completed | error
+    Progress  int                    // 0-100
+    Message   string                 // Current UI message
+    Result    map[string]interface{} // Result when completed
+    Error     string                 // Error text when error
+    CreatedAt time.Time
+    DoneAt    *time.Time
+}
+```
+
+---
+
+## Usage Example (Backend)
+
+```go
+taskID := h.taskManager.Submit(tasks.TaskTypeStockImport,
+    func(ctx context.Context, progressCb func(int, string)) (map[string]interface{}, error) {
+        progressCb(25, "Reading file")
+        // ... work ...
+        progressCb(75, "Saving to DB")
+        return map[string]interface{}{
+            "rows_total": 100,
+            "inserted":   95,
+        }, nil
+    },
+)
+c.JSON(http.StatusOK, gin.H{"task_id": taskID})
+```
+
+---
+
+## API
+
+| Method | URL | Description |
+|--------|-----|-------------|
+| GET | `/api/tasks/:id` | Task status and result |
+
+---
+
+## Implementation
+
+`internal/tasks/` — Task Manager: Submit, track, poll.