feat(worker): fix rating recovery default sigma value

The rating recovery CLI mode (-mode=recalc-ratings) was using
glicko2Tau (0.5) instead of glicko2DefaultSigma (0.06) for the
default sigma value when resetting ratings. This caused the reset
sigma to be ~8x higher than the schema-defined default.

Added glicko2DefaultSigma constant (0.06) and updated ResetAllRatings
and recalcRatings to use it correctly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.needle-predispatch-sha

@@ -1 +1 @@
-2e7eec49516984eb00a641fd03ec7a44fc7212a1
+467b7b67ea1a5b2deab9646070fd3096cb487f9f

cmd/acb-worker/db.go

@@ -710,7 +710,7 @@ func (c *DBClient) ResetAllRatings(ctx context.Context) error {
 	_, err := c.db.ExecContext(ctx, `
 		UPDATE bots
 		SET rating_mu = $1, rating_phi = $2, rating_sigma = $3
-	`, glicko2DefaultMu, glicko2DefaultRD, glicko2Tau)
+	`, glicko2DefaultMu, glicko2DefaultRD, glicko2DefaultSigma)
 	if err != nil {
 		return fmt.Errorf("failed to reset ratings: %w", err)
 	}

cmd/acb-worker/glicko2.go

@@ -5,11 +5,12 @@ package main
 import "math"
 
 const (
-	glicko2Scale     = 173.7178
-	glicko2Tau       = 0.5
-	glicko2DefaultMu = 1500.0
-	glicko2DefaultRD = 350.0
-	glicko2Epsilon   = 1e-6
+	glicko2Scale        = 173.7178
+	glicko2Tau          = 0.5     // Volatility parameter (tau)
+	glicko2DefaultMu    = 1500.0
+	glicko2DefaultRD    = 350.0
+	glicko2DefaultSigma = 0.06    // Default volatility/sigma for new bots
+	glicko2Epsilon      = 1e-6
 )
 
 // Glicko2Rating represents a Glicko-2 rating.

cmd/acb-worker/main.go

@@ -635,7 +635,7 @@ func recalcRatings(ctx context.Context, db *DBClient, logger *log.Logger, verbos
 				currentRatings[p.BotID] = Glicko2Rating{
 					Mu:    glicko2DefaultMu,
 					Phi:   glicko2DefaultRD,
-					Sigma: glicko2Tau, // default sigma
+					Sigma: glicko2DefaultSigma, // default sigma
 				}
 			}
 		}