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/dump.sql

@@ -1497,6 +1497,13 @@ CREATE TABLE template_version_preset_parameters (
     value text NOT NULL
 );
 
+CREATE TABLE template_version_preset_prebuild_schedules (
+    id uuid DEFAULT gen_random_uuid() NOT NULL,
+    preset_id uuid NOT NULL,
+    cron_expression text NOT NULL,
+    desired_instances integer NOT NULL
+);
+
 CREATE TABLE template_version_presets (
     id uuid DEFAULT gen_random_uuid() NOT NULL,
     template_version_id uuid NOT NULL,
@@ -1504,7 +1511,8 @@ CREATE TABLE template_version_presets (
     created_at timestamp with time zone DEFAULT CURRENT_TIMESTAMP NOT NULL,
     desired_instances integer,
     invalidate_after_secs integer DEFAULT 0,
-    prebuild_status prebuild_status DEFAULT 'healthy'::prebuild_status NOT NULL
+    prebuild_status prebuild_status DEFAULT 'healthy'::prebuild_status NOT NULL,
+    scheduling_timezone text DEFAULT ''::text NOT NULL
 );
 
 CREATE TABLE template_version_terraform_values (
@@ -2510,6 +2518,9 @@ ALTER TABLE ONLY template_version_parameters
 ALTER TABLE ONLY template_version_preset_parameters
     ADD CONSTRAINT template_version_preset_parameters_pkey PRIMARY KEY (id);
 
+ALTER TABLE ONLY template_version_preset_prebuild_schedules
+    ADD CONSTRAINT template_version_preset_prebuild_schedules_pkey PRIMARY KEY (id);
+
 ALTER TABLE ONLY template_version_presets
     ADD CONSTRAINT template_version_presets_pkey PRIMARY KEY (id);
 
@@ -2965,6 +2976,9 @@ ALTER TABLE ONLY template_version_parameters
 ALTER TABLE ONLY template_version_preset_parameters
     ADD CONSTRAINT template_version_preset_paramet_template_version_preset_id_fkey FOREIGN KEY (template_version_preset_id) REFERENCES template_version_presets(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY template_version_preset_prebuild_schedules
+    ADD CONSTRAINT template_version_preset_prebuild_schedules_preset_id_fkey FOREIGN KEY (preset_id) REFERENCES template_version_presets(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY template_version_presets
     ADD CONSTRAINT template_version_presets_template_version_id_fkey FOREIGN KEY (template_version_id) REFERENCES template_versions(id) ON DELETE CASCADE;