fix(data): populate base_model_slug on 31,372 smartphone records

[Seungpyo1007]

Summary

TechPicks (reference-consumer app) reported its canonical flagship catalog had base_model_slug null on 100% of records, blocking variant-grouping. Of 93,396 smartphone records, 61,731 already had it (populated on bulk-imported derivative records grouped by physical device); the 31,665 gap was almost entirely the curated/flagship records that never got it computed.

• 12,698 joined an existing same-brand group (12,688 exact slug match, 10 via known import-suffix stripping)
• 18,674 self-referenced (their own slug, since no sibling group exists yet — mostly 2023-2026 flagships with no bulk-import sibling)
• 293 left null on purpose: cross-brand slug collisions (287, e.g. flash exists under alcatel/dell/zte) or multiple same-brand candidate bases (6) — no guessing.

Test plan

• python -m app.validate passed

Closes #1