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/database/dbmem/dbmem.go

@@ -2778,6 +2778,10 @@ func (q *FakeQuerier) GetAPIKeysLastUsedAfter(_ context.Context, after time.Time
 	return apiKeys, nil
 }
 
+func (q *FakeQuerier) GetActivePresetPrebuildSchedules(ctx context.Context) ([]database.TemplateVersionPresetPrebuildSchedule, error) {
+	return nil, ErrUnimplemented
+}
+
 // nolint:revive // It's not a control flag, it's a filter.
 func (q *FakeQuerier) GetActiveUserCount(_ context.Context, includeSystem bool) (int64, error) {
 	q.mutex.RLock()
@@ -9191,6 +9195,15 @@ func (q *FakeQuerier) InsertPresetParameters(_ context.Context, arg database.Ins
 	return presetParameters, nil
 }
 
+func (q *FakeQuerier) InsertPresetPrebuildSchedule(ctx context.Context, arg database.InsertPresetPrebuildScheduleParams) (database.TemplateVersionPresetPrebuildSchedule, error) {
+	err := validateDatabaseType(arg)
+	if err != nil {
+		return database.TemplateVersionPresetPrebuildSchedule{}, err
+	}
+
+	return database.TemplateVersionPresetPrebuildSchedule{}, ErrUnimplemented
+}
+
 func (q *FakeQuerier) InsertProvisionerJob(_ context.Context, arg database.InsertProvisionerJobParams) (database.ProvisionerJob, error) {
 	if err := validateDatabaseType(arg); err != nil {
 		return database.ProvisionerJob{}, err