Document rules and enforce price value uniqueness

modules/purchase_trade/docs/backlog/market-price-import-backlog.md

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+# Backlog - Market Price Import
+
+Scope: code changes related only to `BR-PT-004 - Market Price Import`.
+
+## PT-BL-001 - Enforce uniqueness at model level
+
+- Status: to be tested
+- Priority: high
+- Source: [rules/market-price-import.md](../rules/market-price-import.md)
+- Requirement: enforce uniqueness of `(price, price_date)` at model level without adding a database constraint yet.
+- Work done:
+  - added model-level validation on `price.price_value`
+  - added Tryton message `price.msg_price_value_unique`
+  - added targeted tests for duplicate create and duplicate write validation
+- Validation status:
+  - `git diff --check` passed
+  - automated Python tests still need to be run in a working Tryton Python environment
+- Expected code impact:
+  - `modules/purchase_trade/pricing.py`
+  - possibly `modules/price/price_value.py` if the validation belongs on the target model
+- Expected tests:
+  - importing a row for an existing `(price, price_date)` skips or updates according to `Overwrite existing price`
+  - creating/editing duplicate `price.price_value` records at model level is rejected if validation is implemented on `price.price_value`
+
+## PT-BL-002 - Use locale-aware slash date parsing
+
+- Status: open
+- Priority: medium
+- Source: [rules/market-price-import.md](../rules/market-price-import.md)
+- Requirement: prefer the user's/default locale date format for ambiguous slash dates, then fall back to the current parsing order only if no locale preference is available.
+- Current behavior: `DD/MM/YYYY` is tried before `MM/DD/YYYY`.
+- Expected code impact:
+  - `modules/purchase_trade/pricing.py`
+- Expected tests:
+  - slash dates follow the configured/default locale when available
+  - slash dates keep a deterministic fallback when locale is unavailable
+  - existing accepted formats still work
+
+## PT-BL-003 - Reject missing price_value
+
+- Status: open
+- Priority: high
+- Source: [rules/market-price-import.md](../rules/market-price-import.md)
+- Requirement: `price_value` is mandatory for imported rows. Missing `price_value` must be reported as a row error.
+- Current behavior: empty `price_value` is imported as an empty value.
+- Expected code impact:
+  - `modules/purchase_trade/pricing.py`
+- Expected tests:
+  - missing `price_value` is reported in `errors`
+  - missing `high_price`, `low_price`, and `open_price` remain allowed
+  - valid `price_value` still imports and updates correctly
+
+## PT-BL-004 - Detect duplicate rows inside the same Excel file
+
+- Status: open
+- Priority: high
+- Source: [rules/market-price-import.md](../rules/market-price-import.md)
+- Requirement: duplicate rows for the same `(price_index, price_date)` inside the same Excel file must be reported as row errors. The duplicate row must not import or update data.
+- Expected code impact:
+  - `modules/purchase_trade/pricing.py`
+- Expected tests:
+  - duplicate `(price_index, price_date)` rows in the same import produce row errors
+  - first occurrence behavior remains unchanged
+  - duplicate rows do not overwrite previous imported or updated values
+
+## PT-BL-005 - Categorize import result errors
+
+- Status: open
+- Priority: medium
+- Source: [rules/market-price-import.md](../rules/market-price-import.md)
+- Requirement: distinguish business validation errors from file, parsing, and technical errors in the import result.
+- Current behavior: row errors are grouped under one `Errors` section.
+- Expected code impact:
+  - `modules/purchase_trade/pricing.py`
+  - `modules/purchase_trade/view/import_prices_result_form.xml` only if the result display needs more structure
+- Expected tests:
+  - result message separates business validation errors from parsing/technical errors
+  - invalid workbook and missing required columns remain blocking `UserError`
+  - row-level errors still let the import continue