fix(monitoringV2): cast NUMERIC columns to text in challenge raw query

[TaprootFreak]

Problem

JDM (mainnet) monitoring container has been in a sustained crash with 14+ consecutive failures:

```
ERROR [MonitoringService] Error occurred while processing blocks (14 consecutive failures):
SyntaxError: Cannot convert 2.83917438483003551495e+22 to a BigInt
at ChallengeRepository.findActiveWithChallengePeriod (challenge.repository.js:85:31)
```

This is a release-blocker for #30 Release: develop → main — if it merges as-is, dfxprd will pick up the same crash the moment a single challenge with size > 2^53 wei (~9 JUSD) lands on the mainnet hub.

Root cause

`findActiveWithChallengePeriod` uses `$queryRaw` to select `size` and `current_price` from `challenge_states`. Both columns are `NUMERIC(78,0)`. Without an explicit cast, Prisma's raw query path returned them as JS `number`, which:

1. silently loses precision past `Number.MAX_SAFE_INTEGER` (2^53 - 1)
2. serialises via `.toString()` in scientific notation for very large values

`BigInt()` rejects scientific notation → throw. JDT was unaffected because testnet challenges never crossed 2^53 wei.

Fix

Mirror the pattern already in `position.repository.ts`: explicit `::text` casts in the SQL, typed as `string` in the row shape, then `BigInt(r.size)` directly without an intermediate `.toString()`.

```diff

•   const rows = await this.prisma.\$queryRaw<
  

•   const rows = await this.prisma.\$queryRaw<
    	Array<{
    		...
  

•   		size: any;
  

•   		current_price: any;
  

•   		size: string;
  

•   		current_price: string;
    		...
    	}>
    >\`
    	SELECT c.challenge_id, c.hub_address, c.challenger_address, c.position_address,
  

•   	       c.start_timestamp, c.size, c.current_price, c.t24_alerted, c.t2_alerted,
  

•   	       c.start_timestamp,
  

•   	       c.size::text         as size,
  

•   	       c.current_price::text as current_price,
  

•   	       c.t24_alerted, c.t2_alerted,
    	       p.challenge_period
  

```

Scope

• Bug exists since feat(watchdog): T-24h / T-2h auction deadline escalation alerts #13 (T-24h/T-2h watchdog port from dEURO). dEURO does not have this code path, so no parallel fix needed.
• Other repository methods (`position.repository.ts`, etc.) already use `::text` casts — checked.

Test plan

• After deploy to dfxdev, verify JDM monitoring container processes a full cycle without the `Cannot convert` exception (currently 14 consecutive failures).
• Verify JDT continues to work (no regression).
• Verify deadline watchdog (T-24h / T-2h) still fires correctly — values now arrive as `bigint` instead of via toString-bigint round trip.