feat(database-design): add soft-delete query filters and helpers

[PAMulligan]

Summary

pipeline.config.json enables soft deletes by default (database.softDeleteDefault: true), but no middleware or query-filter implementation existed — generated projects got a deleted_at column with no tooling to use it. This PR adds the full pattern to the templates and the database-design skill.

Changes

• Schema column — nullable deletedAt timestamp added to the shared users schema template (null = active, timestamp = soft-deleted).
• shared/src/db/soft-delete.ts (new) — reusable helpers:
  • notDeleted(table) → WHERE deleted_at IS NULL
  • withSoftDeleteFilter(table, includeDeleted, extra?) → hides deleted rows unless ?include_deleted=true, AND-ing in an optional extra clause
  • softDelete(db, table, id) → stamps deletedAt instead of deleting
  • restore(db, table, id) → clears deletedAt
  • Generic over Drizzle's PgDatabase query-result and schema params, so it works with both the postgres-js (Node) and neon-http (Workers) clients with no casts on the query logic.
• node/src/routes/users.ts (new) — route handler pattern: list/get with ?include_deleted=true, DELETE /:id → softDelete() (404 via .returning()), and POST /:id/restore.
• shared/tests/soft-delete.test.ts (new) — asserts the exact SQL/params each helper produces via Drizzle's .toSQL() (no live DB required).
• database-design skill — new "Step 6.5 — Soft Delete Columns & Helpers" gated on softDeleteDefault; adds the column + index to the Users example, omits deletedAt from write validators, updates the Output table and Design Decisions.

Acceptance criteria

• deleted_at column in the schema template
• Query helper filtering WHERE deleted_at IS NULL
• softDelete() that sets deleted_at instead of deleting
• DELETE route handler pattern calling softDelete()
• ?include_deleted=true admin query param
• database-design skill generates soft-delete columns when enabled
• Integration tests for soft-delete behavior
• CI passes

Verification

• tsc --noEmit passes for both tsconfig.node.json and tsconfig.cloudflare.json (the CI typecheck-templates job).
• No JSON files changed; all tracked JSON still parses.
• Test assertions were confirmed against the installed drizzle-orm@0.45.2 — e.g. softDelete emits update "users" set "deleted_at" = $1 where "users"."id" = $2 with [Date, id]; restore passes [null, id].

Closes #53

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