MongoDB: cap Binary blob size and label FLE/sensitive subtypes (6/7)

Follow-up from PR #129 review (https://github.com/appspace/helix/pull/129#pullrequestreview-4211833511).

## Problem

Today `serializeBinary` in `server/src/drivers/mongodb.ts` base64-encodes the entire buffer, regardless of size. Two concrete hazards:

1. **Size bloat.** A document with a multi-MB GridFS chunk reference, an embedded image, or a subtype-6 ciphertext blob will base64-encode in full and ship over the wire to the grid, where ResultsGrid word-wraps it into a single cell. Easily 10x JSON bloat plus a frozen browser.

2. **Opaque FLE/sensitive subtypes.** Subtype 6 (client-side field-level encryption) and subtype 7 (sensitive) currently render as plain `Binary(6,)`. A user staring at ciphertext won't necessarily realize what they're looking at.

## Proposed shape

- Cap base64 payload at e.g. 1 KB (constant, easy to tune). Beyond the cap, truncate with a length annotation:
`Binary(<sub>,..., bytes)`
- Special-case subtypes 6 and 7 with a label that drops the ciphertext entirely:
`Encrypted( bytes)`</sub>

## Out of scope

Whether to expose a "show full payload" affordance on a row click — that's a UI question for a separate ticket if anyone asks for it.