[simonvanlierde]

Adds CodSpeed benchmarks for the backend code that runs on every upload and every product tree request, measured in CodSpeed's CPU simulation mode. Replaces #462: that one benchmarked the export by validating plain dicts, which the route never does, and its 1280 px test image skipped the JPEG draft step that thumbnail speed depends on.

• Image uploads: metadata stripping with rotation and the thumbnail set, on a 12 MP phone JPEG. Each round gets a fresh copy of the upload, made outside the measurement.
• Product trees: create-request validation, the component tree read at its 1,000-component cap, and the export's read models, JSON and CSV, each through the same functions as its route. Rows are built in memory, so no database is needed.
• Workflow: runs on main and on pull requests that touch the backend, and uploads with OIDC. Only the benchmark job gets id-token: write. CI's test run leaves the benchmarks out.

Tests: just backend/bench (7 passed) and just backend/check. The simulation mode only runs on CodSpeed's runner, so this PR is its first real run.

[codspeed]

Congrats! CodSpeed is installed 🎉

🆕 7 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• test_generate_thumbnails: 1.6 s
• test_process_image_for_storage: 1.6 s
• test_render_products_csv: 229.7 ms
• test_validate_product_create_tree: 8.7 ms
• test_build_product_exports: 580.3 ms
• test_dump_product_exports_json: 210.9 ms
• test_render_component_tree: 347.2 ms

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