epic: SQE backend — serve SQLite databases via SQE alongside other backends

[iheitlager]

Use Case

sqlite-rs as a database backend that SQE serves — SQLite files as data sources, combinable with other backends (Postgres, DuckDB, etc.).

Context

SQE federates multiple database backends. Adding SQLite as a first-class backend means:

• Query SQLite files alongside other data sources
• Use SQLite for local/edge data that joins with remote sources
• No C dependency for SQLite backend support

Requirements

Read path

• Table discovery from SQLite schema
• Type mapping (SQLite affinity → Arrow)
• Pushdown (filters, projections, limits)
• Scan to Arrow batches

Write path (optional)

• INSERT/UPDATE/DELETE via SQE
• Transaction semantics

Federation

• Cross-backend JOINs (SQLite ⋈ Postgres)
• Unified catalog view

Dependencies

• Depends on epic: SQE catalog — replace sqlx-sqlite with sqlite-rs for internal metadata #621 (API facade, bind, Send pager)
• May share virtual table infrastructure (V8)

Path

1. Read-only backend with table discovery + scans
2. Pushdown optimization
3. Write support
4. Federation testing

Success

• sqe query "SELECT * FROM sqlite.my_table JOIN pg.other ..."
• Performance within 2× of direct SQLite access
• Works with existing SQE CLI and connectors