Add get_latest_report tool with equity chart support

**New Tool: get_latest_report**
- Returns the most recent backtest report from the database
- Includes full report details: metrics, paths, tags, notes, verdict
- Optional include_chart parameter (default: true)
- Returns equity chart as base64 PNG when available

**Implementation:**
- ReportDb.get_latest(): Query for most recent report by created_at DESC
- handle_get_latest_report(): Fetches report and reads equity.png from charts_dir
- Base64 encoding using base64 crate v0.22
- Graceful handling when chart not found (equity_chart_error field)

**Files Changed:**
- storage/database.rs: Added get_latest() method
- tools/definitions.rs: Added tool_get_latest_report() definition
- tools/handlers/mod.rs: Added dispatch for get_latest_report
- tools/handlers/reports.rs: Implemented handler with base64 chart encoding
- Cargo.toml: Added base64 = 0.22 dependency

src/tools/definitions.rs

@@ -16,6 +16,7 @@ pub fn get_tools_list() -> Value {
         tool_get_backtest_status(),
         tool_get_optimization_status(),
         tool_prune_reports(),
+        tool_get_latest_report(),
         tool_list_reports(),
         tool_search_reports(),
         tool_tail_log(),
@@ -256,6 +257,19 @@ fn tool_prune_reports() -> Value {
     })
 }
 
+fn tool_get_latest_report() -> Value {
+    json!({
+        "name": "get_latest_report",
+        "description": "Get the latest backtest report with full details and equity chart image",
+        "inputSchema": {
+            "type": "object",
+            "properties": {
+                "include_chart": { "type": "boolean", "description": "Include equity chart as base64 PNG (default: true)", "default": true }
+            }
+        }
+    })
+}
+
 fn tool_list_reports() -> Value {
     json!({
         "name": "list_reports",