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/import-export/contract.md

@@ -20,10 +20,56 @@ Rules:
 
 - User must explicitly choose export scope (`selected`, `filtered`, `all`) when ambiguity exists.
 - Export format should be explicit (`csv`, `json`, etc.).
-- Download response should set:
-  - `Content-Type`
-  - `Content-Disposition`
-- If CSV targets spreadsheet users, document delimiter and BOM policy.
+- Download response must set:
+  - `Content-Type: text/csv; charset=utf-8`
+  - `Content-Disposition: attachment; filename="..."`
+
+## CSV Format Rules (Excel-compatible)
+
+These rules are **mandatory** whenever CSV is exported for spreadsheet users.
+
+### Encoding and BOM
+
+- Write UTF-8 BOM (`\xEF\xBB\xBF`) as the very first bytes of the response.
+- Without BOM, Excel on Windows opens UTF-8 CSV as ANSI and garbles Cyrillic/special characters.
+
+```go
+w.Write([]byte{0xEF, 0xBB, 0xBF})
+```
+
+### Delimiter
+
+- Use **semicolon** (`;`) as the field delimiter, not comma.
+- Excel in Russian/European locale uses semicolon as the list separator.
+- Comma-delimited files open as a single column in these locales.
+
+### Numbers
+
+- Write decimal numbers with a **comma** as the decimal separator: `1 234,56` — not `1234.56`.
+- Excel in Russian locale does not recognize period as a decimal separator in numeric cells.
+- Format integers and floats explicitly; do not rely on Go's default `%v` or `strconv.FormatFloat`.
+- Use a thin non-breaking space (`\u202F`) or regular space as a thousands separator when the value
+  benefits from readability (e.g. prices, quantities > 9999).
+
+```go
+// correct
+fmt.Sprintf("%.2f", price) // then replace "." -> ","
+strings.ReplaceAll(fmt.Sprintf("%.2f", price), ".", ",")
+
+// wrong — produces "1234.56", Excel treats it as text in RU locale
+fmt.Sprintf("%.2f", price)
+```
+
+### Dates
+
+- Write dates as `DD.MM.YYYY` — the format Excel in Russian locale parses as a date cell automatically.
+- Do not use ISO 8601 (`2006-01-02`) for user-facing CSV; it is not auto-recognized as a date in RU locale.
+
+### Text quoting
+
+- Wrap any field that contains the delimiter (`;`), a newline, or a double-quote in double quotes.
+- Escape embedded double-quotes by doubling them: `""`.
+- Use `encoding/csv` with `csv.Writer` and set `csv.Writer.Comma = ';'`; it handles quoting automatically.
 
 ## Error Handling