fix: link the system sqlite instead of bundling it

[clearloop]

Problem

Since 0.5.1 the Arch/AUR build fails to link with undefined symbol: sqlite3_* (reported in #226). 0.5.0 was the last release that built there.

Root cause

0.5.0 had no explicit sqlite dependency, so diesel pulled libsqlite3-sys 0.30.1 non-bundled and linked the system libsqlite3 dynamically. 0.5.1 added rookie, whose rusqlite dragged libsqlite3-sys down to 0.28 with the bundled feature, so sqlite was compiled into a static lib and folded into the rlib. Arch's makepkg link flags (-fuse-ld=lld -Wl,--gc-sections) drop those bundled objects from the final binary, leaving diesel's sqlite3_* references unresolved. The failing 0.5.3 link line confirms it: the bundled sqlite is on the search path with no -lsqlite3, while aws-lc-sys (whole-archived) survives the same flags. #227 then cemented the broken path with an explicit libsqlite3-sys = { version = "0.28", features = ["bundled"] }, which outlived the removal of rookie in #228.

Fix

Drop the explicit libsqlite3-sys pin so diesel resolves a modern libsqlite3-sys (0.37.0) non-bundled, linking the system sqlite again — exactly the configuration that worked in 0.5.0. The README install note now documents the system build dependencies (SQLite + OpenSSL + pkg-config on Linux). Also folds in the native-tls lockfile sync left over from #230 and the 0.5.4 version bump.

Verification

Built clean locally (macOS) and in a Linux container with --all-features under lld -Wl,--gc-sections -Wl,--as-needed. I could not reproduce the original failure in-house because it needs an x86_64 Arch toolchain with the exact makepkg flags, so this restores 0.5.0's known-good config rather than a flag-level patch — final confirmation should come from building 0.5.4 on the AUR.

Tradeoff

Building now requires a system sqlite (plus openssl + pkg-config) on Linux; the bundled zero-config build for bare boxes and Windows is dropped. CI (macOS + Ubuntu) covers the supported platforms.

Addresses #226 (leave open until verified on the AUR).