feat: add storage stats and average latency metrics

Track and display storage and performance metrics that were previously
showing zeros.

Storage metrics:
- Add GetStats() method to storage returning event count and DB size
- Store database file path for size calculation
- Update metrics every 30 seconds via goroutine in main.go
- Display event count and DB size (MB) in dashboard

Performance metrics:
- Calculate average latency from histogram sum/count metrics
- Display as milliseconds in dashboard
- Formula: (duration_sum / duration_count) * 1000

Missing metrics (deferred):
- Connections: requires connection lifecycle tracking (gRPC + WebSocket)
- Deletions: count would need API change to ProcessDeletion

Dashboard now shows accurate storage stats and request latency!

1 files changed, 34 insertions, 2 deletions

diff --git a/internal/storage/storage.go b/internal/storage/storage.go
index 9ef9956..ca9b264 100644
--- a/internal/storage/storage.go
+++ b/internal/storage/storage.go
@@ -4,6 +4,7 @@ import (
         "context"
         "database/sql"
         "fmt"
+        "os"
 
         _ "modernc.org/sqlite" // Pure Go SQLite driver
 )
@@ -11,7 +12,8 @@ import (
 // Storage provides event persistence using SQLite.
 // Consumers should define their own interface based on their needs.
 type Storage struct {
-        db *sql.DB
+        db     *sql.DB
+        dbPath string
 }
 
 // New creates a new Storage instance and initializes the database schema.
@@ -48,7 +50,10 @@ func New(dbPath string) (*Storage, error) {
                 }
         }
 
-        s := &Storage{db: db}
+        s := &Storage{
+                db:     db,
+                dbPath: dbPath,
+        }
 
         // Initialize schema
         if err := s.initSchema(context.Background()); err != nil {
@@ -142,3 +147,30 @@ func (s *Storage) initSchema(ctx context.Context) error {
 func (s *Storage) DB() *sql.DB {
         return s.db
 }
+
+type Stats struct {
+        EventCount int64
+        DBSizeBytes int64
+}
+
+func (s *Storage) GetStats(ctx context.Context) (*Stats, error) {
+        var eventCount int64
+        err := s.db.QueryRowContext(ctx, "SELECT COUNT(*) FROM events WHERE deleted = 0").Scan(&eventCount)
+        if err != nil {
+                return nil, fmt.Errorf("failed to count events: %w", err)
+        }
+
+        var dbSize int64
+        if s.dbPath != ":memory:" {
+                info, err := os.Stat(s.dbPath)
+                if err != nil {
+                        return nil, fmt.Errorf("failed to stat database file: %w", err)
+                }
+                dbSize = info.Size()
+        }
+
+        return &Stats{
+                EventCount: eventCount,
+                DBSizeBytes: dbSize,
+        }, nil
+}