feat(storage): store mongo values as native BSON documents

Implement mongodb_value_codec to encode portfolio/gold/stock values as native
BSON documents instead of JSON-serialized byte strings. Reduces storage size,
improves query efficiency, and eliminates unmarshal overhead. Native BSON
encoding preserves int64 fidelity and enables future native-format queries.
Add NativeValueRepresentation and Int64Fidelity tests validating lossless
encoding. Update migration tool comments. Document native-value behavior
in deploy guide. Update phase-01 plan with implementation notes.

docs/deploy-coolify-selfhosted.md

@@ -63,6 +63,14 @@ and the `STOCK/COIN/GOLD *_API_URL` overrides (modules use coded defaults).
 4. Copy the `mongodb+srv://…` connection string into `MONGO_URL` and put the
    db name in `MONGO_DATABASE`.
 
+> Storage layout: one collection per module; each document is
+> `{ _id: <user key>, value: <native BSON document>, version, updatedAt }`.
+> Values are stored as native BSON (objects/arrays expand and are queryable in
+> Compass); non-JSON values (e.g. a date guard) are plain strings. Concurrency
+> uses the `version` field (optimistic lock). If you migrated data with an older
+> build that stored `value` as a string/blob, re-run the migrator (idempotent)
+> so values become native.
+
 ## 2. Coolify
 
 1. New resource → from this Git repo (Docker Compose), or a prebuilt image.