fix(seed): coerce every *_date field, not just release_date

[Seungpyo1007]

Bug

_load_dir converted ISO date strings by field name:

if isinstance(record.get("release_date"), str):
    record["release_date"] = date.fromisoformat(record["release_date"])

So a category whose date column is named anything else reached SQLite as a str. Seeding the new data/website tree (#48) died on the first insert:

TypeError: SQLite Date type only accepts Python date objects as input
[SQL: INSERT INTO website (slug, name, homepage_url, launch_date, ...)]

Coercion now keys on the _date suffix, so launch_date — and any future category's date column — works without another special case.

Why the existing tests missed it

Every category's endpoint tests insert their fixture through the model, passing a real date object:

session.add(Software(..., release_date=date(1998, 1, 2)))

That never exercises the JSON → DB path where this conversion happens, which is why #48 passed CI with the bug in it. tests/unit/test_seed_date_coercion.py covers that path now: a *_date field is converted, a non-date field is left alone, and a missing directory stays empty.

Verification

• pytest tests/unit/test_seed_date_coercion.py → 3 passed
• Seeded the real 40,084-record data/website tree end to end: SEED OK: {'websites': 40084} (it failed on record 1 before)
• ruff check app tests ✅ · mypy app ✅

Follow-up to #48; the TechAPI data PR is blocked on this.