config + store: remove dead knobs and stale schema

Three drift items surfaced in review, each dead on arrival and each
worth trusting a little more at v0.1.0.

config: drop MetricsPollInterval. The field was parsed from TOML
(metrics_poll_interval), stored on DaemonConfig, and ignored by every
consumer — only StatsPollInterval drives the background poll loop.
Users setting it in config.toml saw zero effect. Removed from the TOML
surface, the model constant, and the config test.

daemon: delete ensureDefaultImage. No callers, body was `_ = ctx;
return nil`. Dead since whatever flow used to call it got removed.

store: drop packages_path from the images table. The column was
carried by the baseline migration but never referenced by UpsertImage
(no INSERT / UPDATE mention) or any Go model field — a ghost from a
build pipeline that no longer exists. Added migration id=2
(drop_dead_image_columns) with an idempotent dropColumnIfExists
helper: fresh installs run baseline (creates the column) + 2 (drops
it); legacy DBs where the column was never added get a no-op. Updated
the direct-INSERT SQL in TestGetImageRejectsMalformedTimestamp to
drop the column reference, and added a migration test covering both
install paths (fresh + legacy).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/model/types.go

@@ -11,18 +11,17 @@ import (
 )
 
 const (
-	DefaultBridgeName                = "br-fc"
-	DefaultBridgeIP                  = "172.16.0.1"
-	DefaultCIDR                      = "24"
-	DefaultDNS                       = "1.1.1.1"
-	DefaultSystemOverlaySize         = 8 * 1024 * 1024 * 1024
-	DefaultWorkDiskSize              = 8 * 1024 * 1024 * 1024
-	DefaultMemoryMiB                 = 2048
-	DefaultVCPUCount                 = 2
-	DefaultStatsPollInterval         = 10 * time.Second
-	DefaultStaleSweepInterval        = 1 * time.Minute
-	DefaultMetricsPollInterval       = 15 * time.Second
-	MaxDiskBytes               int64 = 128 * 1024 * 1024 * 1024
+	DefaultBridgeName               = "br-fc"
+	DefaultBridgeIP                 = "172.16.0.1"
+	DefaultCIDR                     = "24"
+	DefaultDNS                      = "1.1.1.1"
+	DefaultSystemOverlaySize        = 8 * 1024 * 1024 * 1024
+	DefaultWorkDiskSize             = 8 * 1024 * 1024 * 1024
+	DefaultMemoryMiB                = 2048
+	DefaultVCPUCount                = 2
+	DefaultStatsPollInterval        = 10 * time.Second
+	DefaultStaleSweepInterval       = 1 * time.Minute
+	MaxDiskBytes              int64 = 128 * 1024 * 1024 * 1024
 )
 
 type VMState string
@@ -35,20 +34,19 @@ const (
 )
 
 type DaemonConfig struct {
-	LogLevel            string
-	FirecrackerBin      string
-	SSHKeyPath          string
-	AutoStopStaleAfter  time.Duration
-	StatsPollInterval   time.Duration
-	MetricsPollInterval time.Duration
-	BridgeName          string
-	BridgeIP            string
-	CIDR                string
-	TapPoolSize         int
-	DefaultDNS          string
-	DefaultImageName    string
-	FileSync            []FileSyncEntry
-	VMDefaults          VMDefaultsOverride
+	LogLevel           string
+	FirecrackerBin     string
+	SSHKeyPath         string
+	AutoStopStaleAfter time.Duration
+	StatsPollInterval  time.Duration
+	BridgeName         string
+	BridgeIP           string
+	CIDR               string
+	TapPoolSize        int
+	DefaultDNS         string
+	DefaultImageName   string
+	FileSync           []FileSyncEntry
+	VMDefaults         VMDefaultsOverride
 }
 
 // FileSyncEntry is a user-declared host→guest file or directory copy