feat: replace PostgreSQL metrics with SQLite and add metrics query API

Migrate registry metrics from PostgreSQL/sqlx to embedded SQLite via
rusqlite. Add new metrics CLI subcommands (summary, users, downloads)
with i18n support. Record os_version in user activity and package
download requests. Remove old PostgreSQL schema and setup script.

core/Cargo.lock

@@ -1967,6 +1967,18 @@ dependencies = [
  "once_cell",
 ]
 
+[[package]]
+name = "fallible-iterator"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2acce4a10f12dc2fb14a218589d4f1f62ef011b2d0cc4b3cb1bba8e94da14649"
+
+[[package]]
+name = "fallible-streaming-iterator"
+version = "0.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7360491ce676a36bf9bb3c56c1aa791658183a54d2744120f27285738d90465a"
+
 [[package]]
 name = "fastrand"
 version = "2.3.0"
@@ -3562,6 +3574,17 @@ dependencies = [
  "redox_syscall 0.7.3",
 ]
 
+[[package]]
+name = "libsqlite3-sys"
+version = "0.32.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fbb8270bb4060bd76c6e96f20c52d80620f1d82a3470885694e41e0f81ef6fe7"
+dependencies = [
+ "cc",
+ "pkg-config",
+ "vcpkg",
+]
+
 [[package]]
 name = "libyml"
 version = "0.0.5"
@@ -5444,6 +5467,20 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "rusqlite"
+version = "0.34.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "37e34486da88d8e051c7c0e23c3f15fd806ea8546260aa2fec247e97242ec143"
+dependencies = [
+ "bitflags 2.11.0",
+ "fallible-iterator",
+ "fallible-streaming-iterator",
+ "hashlink",
+ "libsqlite3-sys",
+ "smallvec",
+]
+
 [[package]]
 name = "rust-argon2"
 version = "3.0.0"
@@ -6536,6 +6573,7 @@ dependencies = [
  "reqwest_cookie_store",
  "rpassword",
  "rpc-toolkit",
+ "rusqlite",
  "rust-argon2",
  "rust-i18n",
  "semver",