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/dbauthz/dbauthz_test.go

@@ -979,6 +979,29 @@ func (s *MethodTestSuite) TestOrganization() {
 		}
 		check.Args(insertPresetParametersParams).Asserts(rbac.ResourceTemplate, policy.ActionUpdate)
 	}))
+	s.Run("InsertPresetPrebuildSchedule", s.Subtest(func(db database.Store, check *expects) {
+		org := dbgen.Organization(s.T(), db, database.Organization{})
+		user := dbgen.User(s.T(), db, database.User{})
+		template := dbgen.Template(s.T(), db, database.Template{
+			CreatedBy:      user.ID,
+			OrganizationID: org.ID,
+		})
+		templateVersion := dbgen.TemplateVersion(s.T(), db, database.TemplateVersion{
+			TemplateID:     uuid.NullUUID{UUID: template.ID, Valid: true},
+			OrganizationID: org.ID,
+			CreatedBy:      user.ID,
+		})
+		preset := dbgen.Preset(s.T(), db, database.InsertPresetParams{
+			TemplateVersionID: templateVersion.ID,
+			Name:              "test",
+		})
+		arg := database.InsertPresetPrebuildScheduleParams{
+			PresetID: preset.ID,
+		}
+		check.Args(arg).
+			Asserts(rbac.ResourceTemplate, policy.ActionUpdate).
+			ErrorsWithInMemDB(dbmem.ErrUnimplemented)
+	}))
 	s.Run("DeleteOrganizationMember", s.Subtest(func(db database.Store, check *expects) {
 		o := dbgen.Organization(s.T(), db, database.Organization{})
 		u := dbgen.User(s.T(), db, database.User{})
@@ -4916,6 +4939,12 @@ func (s *MethodTestSuite) TestPrebuilds() {
 			Asserts(template.RBACObject(), policy.ActionRead).
 			Returns(insertedParameters)
 	}))
+	s.Run("GetActivePresetPrebuildSchedules", s.Subtest(func(db database.Store, check *expects) {
+		check.Args().
+			Asserts(rbac.ResourceTemplate.All(), policy.ActionRead).
+			Returns([]database.TemplateVersionPresetPrebuildSchedule{}).
+			ErrorsWithInMemDB(dbmem.ErrUnimplemented)
+	}))
 	s.Run("GetPresetsByTemplateVersionID", s.Subtest(func(db database.Store, check *expects) {
 		ctx := context.Background()
 		org := dbgen.Organization(s.T(), db, database.Organization{})