feat: add metabolic age derived value

[DanyPM]

Add Metabolic Age derived value

Summary

Adds Metabolic Age as a new derived measurement value, computed automatically for every measurement that carries a body-composition signal (body-fat % or lean body mass). It follows the existing derived-value pattern (isDerived row, recomputed on every insert/update) alongside BMI, BMR, TDEE, etc.

Scientific basis

Metabolic age answers: at what age would a population-average metabolism match this person's actual metabolism? We derive it by inverting the age term of an established BMR equation:

• Actual BMR — Katch-McArdle: BMR = 370 + 21.6 × FFM(kg). This uses fat-free mass, so it reflects the individual's real body composition (muscle mass), not just body size. FFM is taken from a measured LBM value, or computed as weight × (1 − bodyFat%/100).
• Reference BMR-vs-age — Mifflin-St Jeor: BMR = 10w + 6.25h − 5·age + s, with sex constant s = +5 (male) / −161 (female). This is the population-normative curve at the user's own weight, height and sex, and it is linear in age.
• Metabolic age is the age at which the reference curve equals the actual BMR. Solving the linear equation:

metabolicAge = (10w + 6.25h + s − actualBMR) / 5

A higher actual BMR (more lean mass) yields a younger metabolic age, and vice-versa — the expected physiological direction. The result is clamped to a plausible 15–99 range.

Both formulas are peer-reviewed and already used elsewhere in openScale (Mifflin-St Jeor for BMR; Katch-McArdle in StandardImpedanceLib).

Why a body-composition signal is required

If the "actual" BMR were itself the anthropometric Mifflin-St Jeor value, inverting the curve would trivially return the chronological age. The metric is only meaningful when the actual BMR is derived from fat-free mass. Metabolic age is therefore computed only when the measurement has body-fat % or LBM; plain manual weigh-ins are skipped (and any stale value is removed).

Changes

• New type MeasurementTypeKey.METABOLIC_AGE and unit UnitType.YEARS (displayed as "N years", whole numbers); dedicated hourglass icon.
• Computation DerivedValuesCalculator.processMetabolicAgeCalculation(...) plus fat-free-mass derivation (LBM preferred, else weight + body-fat, with unit normalisation) in the recalculation pass.
• Persistence Room DB version 16 with MIGRATION_15_16 seeding the new derived type; exported schema 16.json.
• Retroactive backfill existing measurements that already carry body-fat/LBM gain a metabolic-age value on next launch — the one-time derived-value backfill gate was widened (and its logging made specific about which values are missing). The check is idempotent and self-terminating.
• Display rendered generically by the existing overview/graph/table screens; UCUM export maps YEARS → "a".

Tests

• DerivedValuesCalculatorFormulasTest — formula correctness (male/female constants, monotonicity: higher FFM → younger age, clamping, null-guards).
• DerivedValuesCalculatorIntegrationTest — end-to-end against a real Room DB: value is persisted from weight + body-fat and removed when the body-composition source disappears.
• MigrationTest — full migration chain (incl. 15→16) produces the new type.