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/migrations/000339_add_scheduling_to_presets.down.sql

@@ -0,0 +1,6 @@
+-- Drop the prebuild schedules table
+DROP TABLE template_version_preset_prebuild_schedules;
+
+-- Remove scheduling_timezone column from template_version_presets table
+ALTER TABLE template_version_presets
+DROP COLUMN scheduling_timezone;

coderd/database/migrations/000339_add_scheduling_to_presets.up.sql

@@ -0,0 +1,12 @@
+-- Add scheduling_timezone column to template_version_presets table
+ALTER TABLE template_version_presets
+ADD COLUMN scheduling_timezone TEXT DEFAULT '' NOT NULL;
+
+-- Add table for prebuild schedules
+CREATE TABLE template_version_preset_prebuild_schedules (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+    preset_id UUID NOT NULL,
+    cron_expression TEXT NOT NULL,
+    desired_instances INTEGER NOT NULL,
+    FOREIGN KEY (preset_id) REFERENCES template_version_presets (id) ON DELETE CASCADE
+);

coderd/database/migrations/testdata/fixtures/000339_add_scheduling_to_presets.up.sql

@@ -0,0 +1,13 @@
+INSERT INTO
+	template_version_preset_prebuild_schedules (
+		id,
+		preset_id,
+		cron_expression,
+		desired_instances
+	)
+	VALUES (
+		'e387cac1-9bf1-4fb6-8a34-db8cfb750dd0',
+		'28b42cc0-c4fe-4907-a0fe-e4d20f1e9bfe',
+		'* 8-18 * * 1-5',
+		1
+	);