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/queries/presets.sql

@@ -5,7 +5,8 @@ INSERT INTO template_version_presets (
 	name,
 	created_at,
 	desired_instances,
-	invalidate_after_secs
+	invalidate_after_secs,
+	scheduling_timezone
 )
 VALUES (
 	@id,
@@ -13,7 +14,8 @@ VALUES (
 	@name,
 	@created_at,
 	@desired_instances,
-	@invalidate_after_secs
+	@invalidate_after_secs,
+	@scheduling_timezone
 ) RETURNING *;
 
 -- name: InsertPresetParameters :many
@@ -25,6 +27,18 @@ SELECT
 	unnest(@values :: TEXT[])
 RETURNING *;
 
+-- name: InsertPresetPrebuildSchedule :one
+INSERT INTO template_version_preset_prebuild_schedules (
+	preset_id,
+	cron_expression,
+	desired_instances
+)
+VALUES (
+	@preset_id,
+	@cron_expression,
+	@desired_instances
+) RETURNING *;
+
 -- name: UpdatePresetPrebuildStatus :exec
 UPDATE template_version_presets
 SET prebuild_status = @status
@@ -69,3 +83,17 @@ SELECT tvp.*, tv.template_id, tv.organization_id FROM
 	template_version_presets tvp
 	INNER JOIN template_versions tv ON tvp.template_version_id = tv.id
 WHERE tvp.id = @preset_id;
+
+-- name: GetActivePresetPrebuildSchedules :many
+SELECT
+	tvpps.*
+FROM
+	template_version_preset_prebuild_schedules tvpps
+		INNER JOIN template_version_presets tvp ON tvp.id = tvpps.preset_id
+		INNER JOIN template_versions tv ON tv.id = tvp.template_version_id
+		INNER JOIN templates t ON t.id = tv.template_id
+WHERE
+	-- Template version is active, and template is not deleted or deprecated
+	tv.id = t.active_version_id
+	AND NOT t.deleted
+	AND t.deprecated = '';