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

Cargo.lock

@@ -94,6 +94,12 @@ version = "1.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c08606f8c3cbf4ce6ec8e28fb0014a2c086708fe954eaa885384a6165172e7e8"
 
+[[package]]
+name = "base64"
+version = "0.22.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b3254f16251a8381aa12e40e3c4d2f0199f8c6508fbecb9d91f575e0fbb8c6"
+
 [[package]]
 name = "bitflags"
 version = "2.11.1"
@@ -478,6 +484,7 @@ name = "mt5-quant"
 version = "1.27.0"
 dependencies = [
  "anyhow",
+ "base64",
  "chrono",
  "clap",
  "dirs",