feat(models): allow null battery_mah/weight_g on mobile devices

[Seungpyo1007]

What & why

Allow unknown battery capacity and weight on tablets, watches, and PDAs to be represented as null instead of fabricated numbers. Accuracy work in GetTechAPI/TechAPI#296 found the same battery_mah=1500 and weight_g=150.0 placeholders across 43 watch and 217 tablet records; the correction in blocked draft GetTechAPI/TechAPI#324 cannot be seeded or dumped while the engine requires numeric values. Those required fields likely encouraged the original placeholder workaround.

How

• Default only mobile-device battery_mah and weight_g to None in SQLModel and the response schema; smartphone requirements and every other field's nullability stay unchanged.
• Allow omitted mobile specs in seed validation while retaining range checks for known values. The serializer, dump generator, and verification consumers already pass through or guard unknown values; battery scoring applies to smartphones.
• Cover omitted values, explicit nulls, each field independently unknown, and known values across tablets, watches, and PDAs through validation, seeding, API responses, and dumps; verify the OpenAPI contract and smartphone requirements.

Existing databases need these columns altered to allow SQL NULL, or the tables recreated, before reseeding corrected records: SQLModel's create_all creates new tables but does not migrate existing ones. This PR changes only TechEngine; it does not modify TechAPI data.

Testing

• pytest --cov=app --cov-report=term --cov-fail-under=60 --tb=short --basetemp=.pytest-nullable-canonical: 552 passed, 77.63% coverage, using TECHAPI_DATA_DIR=C:/Users/29/Desktop/TechAPI/data read-only.
• python -m mypy app: success, 111 source files.
• python -m ruff check app tests: all checks passed.
• python -m app.validate: canonical seed data validation passed.

Checklist

• Type hints on all new functions
• Tests added/updated
• ruff check and pytest pass locally
• Seed-data validation passes; no data changes
• README documents the nullable contract