feat: define canonical AI extraction schema with vision-first prompt and add vision priority verification script

walkthrough.md

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+# Walkthrough: Vision-First Price Extraction
+
+In response to issues where the AI was being misled by hidden text (like unit prices or old prices in HTML), we have implemented a **Vision-Priority Strategy**.
+
+## Changes Implemented
+
+### 1. Updated AI System Prompt (`app/ai_schema.py`)
+
+We completely rewrote the `EXTRACTION_PROMPT_TEMPLATE` to enforce the following rules:
+
+- **Source of Truth = Image**: explicit instruction that the screenshot takes precedence over any text.
+- **Conflict Resolution**: "IF IMAGE AND TEXT CONFLICT, TRUST THE IMAGE."
+- **Visual Focus Rules**:
+  - Look for the largest/boldest price.
+  - Ignore small, styling-less text (often unit prices).
+  - Ignore crossed-out text.
+
+### Verification
+
+We verified the new prompt generation using `verify_vision_priority.py`.
+
+**Generated Prompt Preview:**
+
+```text
+You are a Vision-First Price Extraction Agent.
+Your Goal: Extract the main product price exactly as a human sees it on the screen.
+
+**SOURCE OF TRUTH = IMAGE**
+- The image provided is the **Absolute Truth**.
+- The text provided below is scraped HTML content which may contain hidden/old prices.
+- **IF IMAGE AND TEXT CONFLICT, TRUST THE IMAGE.**
+```
+
+## How to Test
+
+1. Go to "Suivis Prix".
+2. Force refresh an item that was previously incorrect (e.g., B&M item showing unit price).
+3. The AI should now ignore the "hidden" unit price text and read the main price tag from the image.