VFS/DataProvider/Config refactoring + SSH public key authentication

Phase 1-6 of refactoring plan:
- VFS abstraction (VfsBackend trait + LocalFs + OpenFlags builder)
- DataProvider trait (SqliteProvider + PgProvider, SFTPGo-compatible)
- Config refactoring (AppConfig unified sections, env overrides)
- SSH handlers (sftp/scp/rsync) migrated to VFS + DataProvider
- SSH public key authentication (Ed25519 signature verification)
- SSH stderr → CHANNEL_EXTENDED_DATA support
- Web auth uses DataProvider instead of direct SQL
- User home directory from provider (per-user isolation)
- PostgreSQL auth provider for SFTPGo compatibility

Cargo.lock

@@ -2669,6 +2669,7 @@ dependencies = [
  "markbase-webdav",
  "md5 0.8.0",
  "nix 0.29.0",
+ "postgres",
  "pulldown-cmark",
  "rand 0.8.6",
  "regex",
@@ -3678,10 +3679,24 @@ dependencies = [
 ]
 
 [[package]]
-name = "postgres-protocol"
-version = "0.6.11"
+name = "postgres"
+version = "0.19.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "56201207dac53e2f38e848e31b4b91616a6bb6e0c7205b77718994a7f49e70fc"
+checksum = "33ad20e0aa0b24f5a394eab4f78c781d248982b22b25cecc7e3aa46a681605bd"
+dependencies = [
+ "bytes",
+ "fallible-iterator 0.2.0",
+ "futures-util",
+ "log",
+ "tokio",
+ "tokio-postgres",
+]
+
+[[package]]
+name = "postgres-protocol"
+version = "0.6.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "08808e3c483c46e999108051c78334f473d5adb59d78bb80a1268c7e6aa6c514"
 dependencies = [
  "base64",
  "byteorder",
@@ -3697,9 +3712,9 @@ dependencies = [
 
 [[package]]
 name = "postgres-types"
-version = "0.2.13"
+version = "0.2.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8dc729a129e682e8d24170cd30ae1aa01b336b096cbb56df6d534ffec133d186"
+checksum = "851ca9db4932932d69f3ea811b1abe63087a0f740a47692619dd40d4899b68be"
 dependencies = [
  "bytes",
  "fallible-iterator 0.2.0",
@@ -5079,9 +5094,9 @@ dependencies = [
 
 [[package]]
 name = "tokio-postgres"
-version = "0.7.17"
+version = "0.7.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4dd8df5ef180f6364759a6f00f7aadda4fbbac86cdee37480826a6ff9f3574ce"
+checksum = "a528f7d280f6d5b9cd149635c8705b0dd049754bc67d81d31fa25169a93809d3"
 dependencies = [
  "async-trait",
  "byteorder",