fix(pindexer): correct encoding of Amount into Postgres

[cronokirby]

I tried and tried to actually use Numeric directly in a smart way, instead the hack is to just cast text into it in the SQL query. Oh well.

Checklist before requesting a review

• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:

  pindexer bug fix

[conorsch]

Context: while ingesting events from a devnet, we encountered this error in pindexer:

Error: error returned from database: operator does not exist: - bytea

Caused by:
    operator does not exist: - bytea

which we tracked down to

penumbra/crates/bin/pindexer/src/dex/mod.rs

Line 81 in 3b21ac8

VALUES ($1, -$2);

, specifically mismatched types.

[conorsch]

Even with this place in place, when reindexing a devnet db, I encounter a new (but different) error:

2024-07-25T23:24:35.437139Z DEBUG index_event{height=18957 name="penumbra.core.component.dex.v1.EventArbExecution"}: sqlx::query: summary="INSERT INTO trace_step VALUES …" db.statement="\n\nINSERT INTO\n  trace_step\nVALUES\n  (DEFAULT, (CAST($1 AS Amount), $2)) RETURNING id;\n" rows_affected=0 rows_returned=1 elapsed=9.95282ms
2024-07-25T23:24:35.437480Z DEBUG sqlx::query: summary="SELECT events.rowid, events.type, blocks.height …" db.statement="\n\nSELECT\n  events.rowid,\n  events.type,\n  blocks.height AS block_height,\n  tx_results.tx_hash,\n  jsonb_object_agg(attributes.key, attributes.value) AS attrs\nFROM\n  events\n  LEFT JOIN attributes ON events.rowid = attributes.event_id\n  JOIN blocks ON events.block_id = blocks.rowid\n  LEFT JOIN tx_results ON events.tx_id = tx_results.rowid\nWHERE\n  events.rowid > $1\nGROUP BY\n  events.rowid,\n  events.type,\n  blocks.height,\n  tx_results.tx_hash\nORDER BY\n  events.rowid ASC\n" rows_affected=0 rows_returned=1 elapsed=189.203787ms
Error: error occurred while decoding column 0: mismatched types; Rust type `i64` (as SQL type `INT8`) is not compatible with SQL type `INT4`

Caused by:
    mismatched types; Rust type `i64` (as SQL type `INT8`) is not compatible with SQL type `INT4`

which suggests we've still got some typecasting to sort out.