Implement Phase 1: AI-powered categorization with manual review

Add AI categorization service that suggests categories, merchants, and
rules for uncategorized transactions. Users can review and approve
suggestions before applying them.

Features:
- TransactionAICategorizer service using OpenAI GPT-4o-mini
- Batch processing (5 transactions at a time) to avoid rate limits
- Confidence scoring (0-100%) for each suggestion
- AI suggests category, canonical merchant name, and pattern rule
- ReviewAISuggestions page to list uncategorized transactions
- ReviewAISuggestionsWithProposals page for manual review
- Apply individual suggestions or bulk apply high confidence (≥80%)
- Optional rule creation for future auto-categorization
- Cost: ~$0.00015 per transaction (~$0.015 per 100)

CategoryMapping enhancements:
- Confidence field to track AI confidence score
- CreatedBy field ("AI" or "User") to track rule origin
- CreatedAt timestamp for audit trail

Updated ARCHITECTURE.md with complete documentation of:
- TransactionAICategorizer service details
- ReviewAISuggestions page descriptions
- AI categorization workflow (Phase 1)
- Updated CategoryMappings schema

Next steps (Phase 2):
- Auto-apply high confidence suggestions
- Background job processing
- Batch API requests for better efficiency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

ARCHITECTURE.md

@@ -347,6 +347,47 @@ Pattern-based rules for auto-categorization with merchant linking.
 
 **Location:** Services/OpenAIReceiptParser.cs:23-302
 
+### TransactionAICategorizer (Services/TransactionAICategorizer.cs)
+**Interface:** `ITransactionAICategorizer`
+
+**Responsibility:** AI-powered categorization for uncategorized transactions.
+
+**Key Methods:**
+- `ProposeCategorizationAsync(Transaction transaction)`
+  - Analyzes transaction details (name, memo, amount, date)
+  - Calls OpenAI GPT-4o-mini with categorization prompt
+  - Returns `AICategoryProposal` with category, merchant, pattern, and confidence
+  - Auto-suggests rule creation for high confidence (≥70%)
+
+- `ProposeBatchCategorizationAsync(List<Transaction> transactions)`
+  - Processes transactions in batches of 5 to avoid rate limits
+  - Returns list of proposals for review
+
+- `ApplyProposalAsync(long transactionId, AICategoryProposal proposal, bool createRule)`
+  - Updates transaction category and merchant
+  - Optionally creates CategoryMapping rule for future auto-categorization
+  - Returns `ApplyProposalResult` with success status
+
+**API Configuration:**
+- Model: `gpt-4o-mini`
+- Temperature: 0.1 (deterministic)
+- Max tokens: 300
+- API key: Environment variable `OPENAI_API_KEY` or config `OpenAI:ApiKey`
+- Cost: ~$0.00015 per transaction (~$0.015 per 100 transactions)
+
+**Prompt Strategy:**
+- Provides transaction details (name, memo, amount, date)
+- Requests JSON response with category, canonical_merchant, pattern, confidence, reasoning
+- Includes common category examples for context
+- High confidence threshold (≥70%) suggests automatic rule creation
+
+**CategoryMapping Enhancements:**
+- `Confidence` (decimal?) - AI confidence score (0.0-1.0)
+- `CreatedBy` (string?) - "AI" or "User"
+- `CreatedAt` (DateTime?) - Rule creation timestamp
+
+**Location:** Services/TransactionAICategorizer.cs
+
 ## Data Access Layer
 
 ### MoneyMapContext (Data/MoneyMapContext.cs)
@@ -505,6 +546,38 @@ EF Core DbContext managing all database entities.
 
 **Location:** Pages/Recategorize.cshtml.cs:16-87
 
+### ReviewAISuggestions.cshtml / ReviewAISuggestionsModel
+**Route:** `/ReviewAISuggestions`
+
+**Purpose:** AI-powered categorization suggestions for uncategorized transactions.
+
+**Features:**
+- Lists up to 50 most recent uncategorized transactions
+- Generate AI suggestions button (processes up to 20 at a time)
+- Cost transparency (~$0.00015 per transaction)
+- Link to view uncategorized transactions
+
+**Dependencies:** `ITransactionAICategorizer`
+
+**Location:** Pages/ReviewAISuggestions.cshtml.cs
+
+### ReviewAISuggestionsWithProposals.cshtml / ReviewAISuggestionsWithProposalsModel
+**Route:** `/ReviewAISuggestionsWithProposals`
+
+**Purpose:** Review and apply AI categorization proposals.
+
+**Features:**
+- Display AI proposals with confidence scores
+- Color-coded confidence indicators (green ≥80%, yellow 60-79%, red <60%)
+- Individual actions: Accept (with/without rule), Reject, Edit Manually
+- Bulk action: Apply all high-confidence suggestions (≥80%)
+- Shows AI reasoning for each suggestion
+- Stores proposals in session for review workflow
+
+**Dependencies:** `ITransactionAICategorizer`
+
+**Location:** Pages/ReviewAISuggestionsWithProposals.cshtml.cs
+
 ## Configuration
 
 ### appsettings.json
@@ -648,6 +721,9 @@ Category (nvarchar(max), NOT NULL)
 Pattern (nvarchar(max), NOT NULL)
 MerchantId (int, FK → Merchants.Id, SET NULL)
 Priority (int, NOT NULL, DEFAULT 0)
+Confidence (decimal(18,2), NULL) -- AI confidence score
+CreatedBy (nvarchar(max), NULL) -- "AI" or "User"
+CreatedAt (datetime2, NULL) -- Rule creation timestamp
 ```
 
 ## Key Workflows
@@ -740,6 +816,50 @@ Return DashboardData DTO
 Render dashboard view
 ```
 
+### 5. AI-Powered Categorization (Phase 1 - Manual Review)
+
+```
+User visits /ReviewAISuggestions
+    ↓
+ReviewAISuggestionsModel.OnGetAsync()
+    - Loads up to 50 recent uncategorized transactions
+    ↓
+User clicks "Generate AI Suggestions"
+    ↓
+ReviewAISuggestionsModel.OnPostGenerateSuggestionsAsync()
+    - Fetches up to 20 uncategorized transactions
+    - Calls TransactionAICategorizer.ProposeBatchCategorizationAsync()
+    ↓
+For each transaction (batches of 5):
+    TransactionAICategorizer.ProposeCategorizationAsync()
+        - Builds prompt with transaction details
+        - Calls OpenAI GPT-4o-mini API
+        - Parses JSON response
+        - Returns AICategoryProposal
+    ↓
+Store proposals in session
+Redirect to /ReviewAISuggestionsWithProposals
+    ↓
+User reviews proposals with confidence scores
+    ↓
+User actions:
+    Option A: Apply + Create Rule
+        - Updates transaction category and merchant
+        - Creates CategoryMapping rule (CreatedBy="AI")
+        - Future similar transactions auto-categorized
+    Option B: Apply (No Rule)
+        - Updates transaction only
+        - No rule created
+    Option C: Reject
+        - Removes proposal from session
+    Option D: Edit Manually
+        - Redirects to EditTransaction page
+    ↓
+Proposal applied
+Remove from session
+Display success message
+```
+
 ## Design Patterns
 
 ### 1. Service Layer Pattern