Switch BDK persistence from file store to SQLite

[markuslevonyak]

rgb-lib currently persists BDK wallet data with bdk_file_store, which upstream explicitly documents as unfit for production:

bdk_file_store is a development/testing database. It does not natively support backwards compatible BDK version upgrades so should not be used in production.

This already caused breakage: the file store format changed in the BDK upgrade from 2.0 to 3.0, leaving existing databases unreadable. SQLite is the production-oriented, upgrade-migratable backend.

This PR moves BDK persistence to bdk_sqlite (BDK's sqlx-based SQLite store). Using it rather than bdk_wallet's built-in rusqlite feature also avoids a native-library conflict: rusqlite and rgb-lib's existing sea-orm/sqlx stack pull different libsqlite3-sys versions, and two crates linking sqlite3 can't coexist. bdk_sqlite shares sea-orm's sqlx, so the dependency graph resolves cleanly.

Note on existing wallets

This switch (like the BDK 2.0 to 3.0 upgrade before it) is not backwards compatible: existing BDK file-store databases are not read, so on first open the wallet creates a fresh SQLite store and repopulates via the normal scan. This is unavoidable when changing persistence backends and is the motivation for moving to a properly migratable one going forward.

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.73%. Comparing base (1e6353d) to head (6568548).

Files with missing lines	Patch %	Lines
src/wallet/core.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   95.74%   95.73%   -0.02%     
==========================================
  Files          23       23              
  Lines       12835    12818      -17     
==========================================
- Hits        12289    12271      -18     
- Misses        546      547       +1     

Flag	Coverage Δ
rust	95.73% <96.15%> (-0.02%)	⬇️

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[zoedberg]

Thank you for this PR, we really appreciate the detailed explanation of the dependency and migration tradeoffs.
FYI, I've added a warning about the BDK storage backwards compatibility issue to the 0.3.0 beta 6 release notes here.
At the moment, we don't have the bandwidth to review this properly, but we will take a closer look in a couple of weeks. We'll get back to you as soon as we can. Thanks for your patience and contribution!