chore(db): CHECK constraint for field_corrections.item_index

[Fluory]

Ziel

The database itself rejects impossible line-item indexes in field_corrections.item_index (added in migration 0014, PR #42). Today only the review service validates the index before the insert.

Akzeptanzkriterien

• A new migration adds CHECK (item_index IS NULL OR item_index >= 0) on app.field_corrections.
• Inserting a negative item_index as app_rw fails with a constraint violation.
• Existing rows pass the constraint (migration applies on a DB migrated 0000 → latest with the test data).
• docs/technical/data-model.md lists the constraint.

Nicht Teil dieser Aufgabe

• An upper bound tied to the number of extracted line items (stays in the app – it depends on the latest run).

Betroffene Bereiche

• src/db/schema/app.ts, new migration under src/db/migrations/, docs/technical/data-model.md

Testplan

Kriterium	Prüfart
Negative index rejected	integration test (real Postgres)
Migration applies on existing data	pnpm verify on a fresh DB + migration test

Security/Privacy betroffen?

Nein.

Deliberately not done in PR #42 so the stacked migration numbers (0014/0015) stayed stable. Must be created after the stack #21 … #45 is merged.

[Fluory]

Claimed by @Fluory (Claude Code session) on branch claude/chore-item-index-check-47. Draft PR follows.

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