refactor: BTC/LTC are Electrum/Fulcrum-only (drop Core-RPC client + dep)

Electrum/Fulcrum already covers everything the backend needs for Bitcoin
and Litecoin — balance, UTXOs, history, tip, verbose tx, fee estimation,
and broadcast (blockchain.transaction.broadcast). A self-hosted Fulcrum
relays broadcasts to its own backing node, so a separate Bitcoin Core
RPC path buys nothing in the self-host model while adding the
bitcoincore-rpc + bitcoin + secp256k1 + jsonrpc dependency tree and a
duplicate broadcast surface.

- Remove the `bitcoincore-rpc` dependency (and its transitive tree).
- Slim `UtxoConfig` to pure Electrum config: drop use_local_node /
  rpc_url / rpc_user / rpc_pass; keep network + electrum_primary +
  electrum_fallbacks.

A future LTC-MWEB or direct-node provider is a distinct capability
(MWEB uses a scan-key model, not canonical scriptPubKeys) and would slot
in at the provider seam rather than extending this client — so nothing is
foreclosed by keeping the surface minimal now.

Cargo.lock

@@ -270,12 +270,6 @@ dependencies = [
  "thiserror 1.0.69",
 ]
 
-[[package]]
-name = "base64"
-version = "0.13.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9e1b586273c5702936fe7b7d6896644d8be71e6314cfe09d3167c95f712589e8"
-
 [[package]]
 name = "base64"
 version = "0.21.7"
@@ -294,76 +288,12 @@ version = "1.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2af50177e190e07a26ab74f8b1efbfe2ef87da2116221318cb1c2e82baf7de06"
 
-[[package]]
-name = "bech32"
-version = "0.10.0-beta"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "98f7eed2b2781a6f0b5c903471d48e15f56fb4e1165df8a9a2337fd1a59d45ea"
-
 [[package]]
 name = "bech32"
 version = "0.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "efbd3e1070bbdf4cd88a75264e18e8a26f7cb5c6949eadf0ceb85fb159cf08f8"
 
-[[package]]
-name = "bitcoin"
-version = "0.31.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "69197dee21fe23b45f5239bf88086efaa0cb8679f3e704906eb818e8ea169c14"
-dependencies = [
- "bech32 0.10.0-beta",
- "bitcoin-internals",
- "bitcoin_hashes",
- "hex-conservative",
- "hex_lit",
- "secp256k1",
- "serde",
-]
-
-[[package]]
-name = "bitcoin-internals"
-version = "0.2.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "994dc6fcc13751c85370b7de118e672b193b9b65167bf09e258f124c97fb9685"
-dependencies = [
- "serde",
-]
-
-[[package]]
-name = "bitcoin_hashes"
-version = "0.13.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "446819536d8121575eeb7e89efdbadb3f055e87e4bb66c6679a6d5cc2f4b64fd"
-dependencies = [
- "hex-conservative",
- "serde",
-]
-
-[[package]]
-name = "bitcoincore-rpc"
-version = "0.18.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8eb70725a621848c83b3809913d5314c0d20ca84877d99dd909504b564edab00"
-dependencies = [
- "bitcoincore-rpc-json",
- "jsonrpc",
- "log",
- "serde",
- "serde_json",
-]
-
-[[package]]
-name = "bitcoincore-rpc-json"
-version = "0.18.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "856ffbee2e492c23bca715d72ea34aae80d58400f2bda26a82015d6bc2ec3662"
-dependencies = [
- "bitcoin",
- "serde",
- "serde_json",
-]
-
 [[package]]
 name = "bitflags"
 version = "1.3.2"
@@ -1183,24 +1113,12 @@ version = "0.4.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
 
-[[package]]
-name = "hex-conservative"
-version = "0.1.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "212ab92002354b4819390025006c897e8140934349e8635c9b077f47b4dcbd20"
-
 [[package]]
 name = "hex-literal"
 version = "0.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6fe2267d4ed49bc07b63801559be28c718ea06c4738b7a03c94df7386d2cde46"
 
-[[package]]
-name = "hex_lit"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3011d1213f159867b13cfd6ac92d2cd5f1345762c63be3554e84092d85a50bbd"
-
 [[package]]
 name = "hkdf"
 version = "0.12.4"
@@ -1577,17 +1495,6 @@ dependencies = [
  "wasm-bindgen",
 ]
 
-[[package]]
-name = "jsonrpc"
-version = "0.14.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8128f36b47411cd3f044be8c1f5cc0c9e24d1d1bfdc45f0a57897b32513053f2"
-dependencies = [
- "base64 0.13.1",
- "serde",
- "serde_json",
-]
-
 [[package]]
 name = "jsonwebtoken"
 version = "9.3.1"
@@ -2592,27 +2499,6 @@ dependencies = [
  "zeroize",
 ]
 
-[[package]]
-name = "secp256k1"
-version = "0.28.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d24b59d129cdadea20aea4fb2352fa053712e5d713eee47d700cd4b2bc002f10"
-dependencies = [
- "bitcoin_hashes",
- "rand",
- "secp256k1-sys",
- "serde",
-]
-
-[[package]]
-name = "secp256k1-sys"
-version = "0.9.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e5d1746aae42c19d583c3c1a8c646bfad910498e2051c551a7f2e3c0c9fbb7eb"
-dependencies = [
- "cc",
-]
-
 [[package]]
 name = "security-framework"
 version = "3.7.0"
@@ -2805,8 +2691,7 @@ dependencies = [
  "axum",
  "axum-extra",
  "base64 0.22.1",
- "bech32 0.12.0",
- "bitcoincore-rpc",
+ "bech32",
  "bs58",
  "chrono",
  "dotenvy",