feat(db): add zstd compression to Transactions table

[kariy]

Extracts the envelope logic introduced in #465 for ReceiptEnvelope into a generic Envelope<T> parameterized by an EnvelopePayload trait, then applies the same zstd compression to the Transactions table via a new TxEnvelope type alias.

Each payload type supplies a 4-byte magic (KRCP for receipts, KTXN for transactions), a human-readable name for error messages, and a from_legacy_bytes function for backward-compatible deserialization of pre-envelope rows. The shared Compress/Decompress impls handle the header, zstd encode/decode, version/encoding validation, and legacy fallback.

The Transactions table value type changes from VersionedTx to TxEnvelope. Existing uncompressed rows are transparently decoded via the legacy fallback in VersionedTx::from_legacy_bytes, so no migration is needed. However, any code that reads directly from the Transactions table outside of the provider (eg. custom tooling) will need to update to handle TxEnvelope instead of bare VersionedTx.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 92.02899% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.54%. Comparing base (9bde0ae) to head (b1b3499).
⚠️ Report is 306 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/storage/db/src/models/envelope.rs	93.13%	7 Missing ⚠️
...storage/db/src/models/versioned/transaction/mod.rs	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #477      +/-   ##
==========================================
- Coverage   73.32%   68.54%   -4.78%     
==========================================
  Files         209      281      +72     
  Lines       23132    34394   +11262     
==========================================
+ Hits        16961    23577    +6616     
- Misses       6171    10817    +4646     

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