Prevent parallel docs model generation from overwriting shared output

[coderabbitai]

Summary

make -j docs/models can execute the phony PlantUML model targets concurrently. Each underlying pyreverse command writes its intermediate result to the same docs/models/classes.puml path before it is renamed, so diagrams can be overwritten, mismatched, or cause a target to fail.

Required changes

Update the documentation model generation rules in Makefile so each of these targets has isolated intermediate output or is explicitly serialized:

• docs/models/requests.puml
• docs/models/responses.puml
• docs/models/common.puml
• docs/models/database.puml

Preserve the existing final generated filenames and the existing docs/models interface.

Rationale

The model targets are declared .PHONY, which allows GNU Make to schedule them in parallel. Sharing docs/models/classes.puml makes the recipes unsafe under parallel execution.

Affected area

• Makefile documentation model-generation targets

Acceptance criteria

• Running make -j docs/models does not share or overwrite an intermediate classes.puml file between targets.
• Each target produces its matching final .puml diagram.
• The target behavior remains compatible with the SVG generation targets.

Backlinks

• Reported from PR LCORE-3141: Docs models are PHONY targets #2225: LCORE-3141: Docs models are PHONY targets #2225
• Review comment: LCORE-3141: Docs models are PHONY targets #2225 (comment)
• Requested by @tisnik.