feat(features): implement lifecycle compute method (#109)

[w7-mgfcode]

Summary

Lands PRP-3.1B — the lifecycle compute slice for time-safe featuresets.

• Adds FeatureEngineeringService._compute_lifecycle_features (private method).
• Wires it at step 6 of compute_features, gated on self.config.lifecycle_config.
• Defensive no-op when launch_date / discontinue_date columns are absent from the input DataFrame — emits an info log and returns zero new columns. End-to-end loader extension (joining product attrs) lands in PRP-3.1E.
• Two features when fully configured:
  • days_since_launch_lag{N} (lagged per (store_id, product_id))
  • days_since_discontinue_lag{N} (signed delta — LightGBM learns the sign; do not clip)

Time-safety

• Uses the canonical groupby(entity).shift(lag) idiom — row i reflects only data at row i - lag.
• No changes to test_leakage.py invariants for the existing families.

Scope

• Files touched: service.py, tests/test_leakage.py, tests/test_service.py (3 files, +304/-0).
• No schemas.py / routes.py / conftest.py / migration / docs edits.
• Additive-contract snapshot (config_hash_unchanged_when_phase2_omitted) still passes.

Test plan

• ruff check + ruff format --check
• mypy --strict app/
• pyright --strict app/
• pytest -v on the slice (82 tests pass)
• CI green on PR

Refs

Part of #109 (Phase 2 feature wiring). PRP-3.1E will land the loader extension that actually populates launch_date / discontinue_date from the product table.

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