feat: implement scheduling mechanism for prebuilds (#18126)

Closes https://github.com/coder/internal/issues/312
Depends on https://github.com/coder/terraform-provider-coder/pull/408

This PR adds support for defining an **autoscaling block** for
prebuilds, allowing number of desired instances to scale dynamically
based on a schedule.

Example usage:
```
data "coder_workspace_preset" "us-nix" {
  ...
  
  prebuilds = {
    instances = 0                  # default to 0 instances
    
    scheduling = {
      timezone = "UTC"             # a single timezone is used for simplicity
      
      # Scale to 3 instances during the work week
      schedule {
        cron = "* 8-18 * * 1-5"    # from 8AM–6:59PM, Mon–Fri, UTC
        instances = 3              # scale to 3 instances
      }
      
      # Scale to 1 instance on Saturdays for urgent support queries
      schedule {
        cron = "* 8-14 * * 6"      # from 8AM–2:59PM, Sat, UTC
        instances = 1              # scale to 1 instance
      }
    }
  }
}
```

### Behavior
- Multiple `schedule` blocks per `prebuilds` block are supported.
- If the current time matches any defined autoscaling schedule, the
corresponding number of instances is used.
- If no schedule matches, the **default instance count**
(`prebuilds.instances`) is used as a fallback.

### Why
This feature allows prebuild instance capacity to adapt to predictable
usage patterns, such as:
- Scaling up during business hours or high-demand periods
- Reducing capacity during off-hours to save resources

### Cron specification
The cron specification is interpreted as a **continuous time range.**

For example, the expression:

```
* 9-18 * * 1-5
```

is intended to represent a continuous range from **09:00 to 18:59**,
Monday through Friday.

However, due to minor implementation imprecision, it is currently
interpreted as a range from **08:59:00 to 18:58:59**, Monday through
Friday.

This slight discrepancy arises because the evaluation is based on
whether a specific **point in time** falls within the range, using the
`github.com/coder/coder/v2/coderd/schedule/cron` library, which performs
per-minute matching rather than strict range evaluation.

---------

Co-authored-by: Danny Kopping <danny@coder.com>

coderd/provisionerdserver/provisionerdserver.go

@@ -2197,7 +2197,13 @@ func InsertWorkspacePresetsAndParameters(ctx context.Context, logger slog.Logger
 
 func InsertWorkspacePresetAndParameters(ctx context.Context, db database.Store, templateVersionID uuid.UUID, protoPreset *sdkproto.Preset, t time.Time) error {
 	err := db.InTx(func(tx database.Store) error {
-		var desiredInstances, ttl sql.NullInt32
+		var (
+			desiredInstances   sql.NullInt32
+			ttl                sql.NullInt32
+			schedulingEnabled  bool
+			schedulingTimezone string
+			prebuildSchedules  []*sdkproto.Schedule
+		)
 		if protoPreset != nil && protoPreset.Prebuild != nil {
 			desiredInstances = sql.NullInt32{
 				Int32: protoPreset.Prebuild.Instances,
@@ -2209,6 +2215,11 @@ func InsertWorkspacePresetAndParameters(ctx context.Context, db database.Store,
 					Valid: true,
 				}
 			}
+			if protoPreset.Prebuild.Scheduling != nil {
+				schedulingEnabled = true
+				schedulingTimezone = protoPreset.Prebuild.Scheduling.Timezone
+				prebuildSchedules = protoPreset.Prebuild.Scheduling.Schedule
+			}
 		}
 		dbPreset, err := tx.InsertPreset(ctx, database.InsertPresetParams{
 			ID:                  uuid.New(),
@@ -2217,11 +2228,25 @@ func InsertWorkspacePresetAndParameters(ctx context.Context, db database.Store,
 			CreatedAt:           t,
 			DesiredInstances:    desiredInstances,
 			InvalidateAfterSecs: ttl,
+			SchedulingTimezone:  schedulingTimezone,
 		})
 		if err != nil {
 			return xerrors.Errorf("insert preset: %w", err)
 		}
 
+		if schedulingEnabled {
+			for _, schedule := range prebuildSchedules {
+				_, err := tx.InsertPresetPrebuildSchedule(ctx, database.InsertPresetPrebuildScheduleParams{
+					PresetID:         dbPreset.ID,
+					CronExpression:   schedule.Cron,
+					DesiredInstances: schedule.Instances,
+				})
+				if err != nil {
+					return xerrors.Errorf("failed to insert preset prebuild schedule: %w", err)
+				}
+			}
+		}
+
 		var presetParameterNames []string
 		var presetParameterValues []string
 		for _, parameter := range protoPreset.Parameters {