Add shared engineering rule contracts

- go-logging: slog, server-side only, structured attributes
- go-database: MySQL cursor safety, soft delete, GORM tags, fail-fast, N+1 prevention
- go-background-tasks: Task Manager pattern, polling, no SSE
- go-code-style: layering, error wrapping, startup sequence, config, templating
- import-export: CSV Excel-compatible rules (BOM, semicolon, decimal comma, DD.MM.YYYY)
- table-management: filtering and pagination rules added
- CLAUDE.template.md: updated to reference all shared contracts

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

kit/patterns/go-background-tasks/contract.md

@@ -0,0 +1,73 @@
+# Contract: Background Tasks (Go Web Applications)
+
+## Core Rule
+
+All long-running operations (> ~300ms or uncertain duration) must run as background tasks.
+Never block an HTTP handler waiting for a slow operation to complete.
+
+## Standard Flow
+
+```
+POST /api/something → creates task → returns {task_id}
+GET  /api/tasks/:id → client polls → returns status + progress
+UI   shows spinner → on success: toast + refresh
+```
+
+The client polls `/api/tasks/:id` on a fixed interval (1–3 s) until status is terminal.
+
+## Task Status Values
+
+| Status      | Meaning |
+|-------------|---------|
+| `queued`    | Created, not yet started |
+| `running`   | In progress |
+| `success`   | Completed successfully |
+| `failed`    | Completed with error |
+| `canceled`  | Explicitly canceled by user |
+
+## Task Struct (canonical fields)
+
+```go
+type Task struct {
+    ID       string
+    Type     string    // e.g. "recalculate", "import", "export"
+    Status   string
+    Progress int       // 0–100
+    Message  string    // human-readable current step
+    Result   any       // populated on success (row count, file URL, etc.)
+    Error    string    // populated on failure
+    CreatedAt time.Time
+    UpdatedAt time.Time
+}
+```
+
+## Rules
+
+- Do NOT use SSE (Server-Sent Events) for task progress. Use polling.
+  SSE requires persistent connections and complicates load balancers and proxies.
+- One task per operation. Do not batch unrelated operations into a single task.
+- Tasks must be stored in memory (or DB for persistence) with a TTL — clean up after completion.
+- Handler that creates the task returns `202 Accepted` with `{"task_id": "..."}`.
+- Log task start, finish, and error server-side (see go-logging contract).
+
+## UI Contract
+
+- Show a spinner/progress bar while `status == "running"`.
+- On `success`: show toast notification, refresh affected data.
+- On `failed`: show error message from `task.Error`.
+- Poll interval: 1–3 seconds. Stop polling on terminal status.
+- Do not disable the whole page during task execution — allow user to navigate away.
+
+## What Qualifies as a Background Task
+
+- CSV/file export with > ~500 rows
+- Bulk recalculation or recompute operations
+- External API calls (sync, collect, import from remote)
+- Any import/ingest that touches multiple DB tables
+- Database repair or cleanup operations
+
+## What Does NOT Need a Background Task
+
+- Simple CRUD operations (create/update/delete single record)
+- Filtered list queries
+- Single-record exports