Optimizer: hourly slots beyond 48h also for a self-hosted optimizer

[Thomvh]

fixes #34820, follows #34738

As agreed in the issue, the horizon beyond 48h is now merged into clock hour slots for every optimizer, not only the hosted one. Local installs can opt out or move the boundary with an environment variable.

• coarsenHorizon runs for hosted and self-hosted optimizers, default boundary 48h
• new env OPTIMIZER_COARSEN_AFTER (whole hours, e.g. 72), only honoured by a self-hosted optimizer; 0 keeps quarter hour slots for the full horizon
• an invalid or negative value logs a warning and falls back to 48h
• behaviour change for existing self-hosted users: long horizons now use hourly slots unless they set OPTIMIZER_COARSEN_AFTER=0

TODO

• document OPTIMIZER_COARSEN_AFTER next to OPTIMIZER_URI in the docs: Optimizer: document OPTIMIZER_COARSEN_AFTER docs#1221

🤖 Generated with Claude Code

[sourcery-ai]

Hey - I've found 2 issues

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="core/site_optimizer.go" line_range="1388" />
<code_context>
+// coarseSlotsAfter returns where the request switches to clock hour slots. A self-hosted optimizer
+// may move it with OPTIMIZER_COARSEN_AFTER, 0 keeps quarter hour slots for the full horizon.
+func coarseSlotsAfter(hosted bool) (time.Duration, error) {
+	v := os.Getenv("OPTIMIZER_COARSEN_AFTER")
+	if hosted || v == "" {
+		return defaultCoarseSlotsAfter, nil
</code_context>
<issue_to_address>
**Self-hosted users cannot discover override**

When a self-hosted user relies on the optimizer documentation to configure options, the optimizer documentation omits `OPTIMIZER_COARSEN_AFTER`, so users cannot learn how to set it to `0` to preserve quarter-hour slots or move the boundary, and requests retain hourly slots beyond 48 hours unintentionally.

Document `OPTIMIZER_COARSEN_AFTER`, including that `0` preserves quarter-hour slots for the full horizon and how to move the boundary.
</issue_to_address>

### Comment 2
<location path="core/site_optimizer_test.go" line_range="238-239" />
<code_context>
 	after := before
-	coarsenHorizon(&short, &after)
+	coarsenHorizon(&short, &after, defaultCoarseSlotsAfter)
 	assert.Equal(t, 4*47, len(short.TimeSeries.Dt))
 	assert.Equal(t, before, after)
+
+	// an earlier boundary coarsens the same request
</code_context>
<issue_to_address>
**Short-horizon mutations go undetected**

When the coarsener changes slot durations in place without changing the slot count, when the coarsener changes `short.TimeSeries.Dt` values without changing its length, `TestCoarsenHorizon` line 238 still passes and line 239 never compares `Dt`; the test passes despite a short-horizon mutation.

Snapshot and compare `short.TimeSeries.Dt` before and after coarsening.
</issue_to_address>

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[sourcery-ai]

🟡 Medium · Self-hosted users cannot discover override

When a self-hosted user relies on the optimizer documentation to configure options, the optimizer documentation omits OPTIMIZER_COARSEN_AFTER, so users cannot learn how to set it to 0 to preserve quarter-hour slots or move the boundary, and requests retain hourly slots beyond 48 hours unintentionally.

Document OPTIMIZER_COARSEN_AFTER, including that 0 preserves quarter-hour slots for the full horizon and how to move the boundary.

Prompt for AI agents

In `core/site_optimizer.go` at line 1388:

**Self-hosted users cannot discover override**

When a self-hosted user relies on the optimizer documentation to configure options, the optimizer documentation omits `OPTIMIZER_COARSEN_AFTER`, so users cannot learn how to set it to `0` to preserve quarter-hour slots or move the boundary, and requests retain hourly slots beyond 48 hours unintentionally.

Document `OPTIMIZER_COARSEN_AFTER`, including that `0` preserves quarter-hour slots for the full horizon and how to move the boundary.