docs(getting-started): add sqlite build step, fix init error

[TheWitcherish]

Fixes #405.

• Adds the missing cargo build --release --no-default-features --features sqlite line to the "Build extenddb" section of docs/getting-started.md, and cross-references it from the "SQLite backend (developer mode)" section.
• Fixes the init --backend mismatch error in crates/app/src/cmd_init.rs to name the actual rebuild command instead of a nonexistent extenddb-<backend> binary.

Tested: default build + extenddb init --backend sqlite now prints the corrected error; cargo build --release --no-default-features --features sqlite + extenddb init --backend sqlite exits 0 as documented. cargo fmt --check and a plain cargo build --release both pass; pre-existing clippy -D warnings noise in crates/auth (async_trait macro, unrelated to this change) reproduces identically on main.