fix: backfill missing request_stats columns on existing databases

[marcuslannister]

Summary

Databases created before the trace_id and timing_spans columns existed on request_stats never got them, because the schema init uses CREATE TABLE IF NOT EXISTS — a no-op when the table already exists:

• On SQLite this crashed the service at startup: CREATE INDEX ix_request_stats_trace_id ON request_stats(trace_id) fails outright when the column doesn't exist.
• On Postgres (and SQLite once past startup) it silently failed every insert into request_stats with timing_spans set (logged as rust_persistence_write_error, not fatal, but stats stopped recording).

Hit this in production on an upgrade from an older image against a pre-existing stats.db.

Changes

• Split each backend's schema constant into *_SCHEMA_TABLES / *_SCHEMA_INDEXES, and run a column-backfill migration in between table creation and index creation.
• REQUEST_STATS_COLUMNS is a single (name, sqlite_type, postgres_type) list shared by both backends — adding a future column only needs one entry here instead of a new one-off ALTER TABLE in two places.
• Both migrate_sqlite_columns and migrate_postgres_columns check existing columns first (PRAGMA table_info / information_schema.columns) and only issue ALTER TABLE ADD COLUMN for what's actually missing. This matters for Postgres in particular: an unconditional ALTER TABLE ... ADD COLUMN IF NOT EXISTS still requires ALTER privilege even when the column already exists, which would break startup for a least-privilege application role on an already-migrated database.
• Pure additive ALTER TABLE ADD COLUMN, no drops/rewrites — no data loss.

Test plan

• cargo test --bin uni-api-front persistence — existing tests pass, plus a new test covering backfill from an old schema and idempotency on re-run.
• cargo clippy --bin uni-api-front — clean on this file.
• Verified against a real production SQLite database that predated both columns: migration backfills, service starts, and subsequent requests write stats without error.