feat: add SQLite support behind a sqlite Cargo feature

Adds a SqliteDialect alongside PostgresDialect, gated by mutually exclusive
postgres (default) / sqlite Cargo features, plus a feature-selected
ActiveDatabase type alias used throughout the public Georm/Defaultable
traits. Basic CRUD and Defaultable are fully covered by a new SQLite test
suite (tests/sqlite/*) running against a real SQLite database, alongside
split migrations/sqlite/*.

Two real dialect differences surfaced beyond placeholder syntax:
- SQLite's INTEGER columns type-infer as i64 in sqlx's compile-time query
  macros (unlike Postgres SERIAL -> i32), so the SQLite test models use i64
  for integer PK/FK fields.
- sqlx's SQLite query macros need bind arguments bound to a place rather
  than passed as an inline expression (fixed once, centrally, in
  SqlDialect::generate_relation_lookup).

Composite-key entities with chrono::DateTime columns (tests/composite_key.rs)
are not yet ported to SQLite: SQLite has no TIMESTAMPTZ equivalent, so sqlx
infers TEXT date/time columns as String rather than DateTime<Utc> unless the
query explicitly overrides the column type. That needs per-field type
overrides threaded through the generated queries, left as follow-up.

CI now matrixes over both backends; formatting/audit only run once since
they don't vary by backend. examples/postgres/* stays out of
--no-default-features builds by scoping sqlite lint/test just recipes to
`-p georm` (it depends on georm's default postgres feature via Cargo
feature unification, so building it under --features sqlite at the
workspace level double-activates both dialects).

justfile

@@ -3,18 +3,36 @@ default: lint
 clean:
 	cargo clean
 
-test:
+# Runs the whole workspace (examples included) against the default
+# "postgres" feature.
+test: test-postgres
+
+test-postgres:
 	cargo test --all-targets --all
 
+# Scoped to -p georm: examples/postgres/* depends on georm's default
+# ("postgres") feature via Cargo feature unification, so it must stay out of
+# a --no-default-features build rather than be dragged into it.
+test-sqlite:
+	cargo test -p georm --no-default-features --features sqlite --all-targets
+
 lint:
 	cargo clippy --all-targets
 
+lint-sqlite:
+	cargo clippy -p georm -p georm-macros --no-default-features --features sqlite --all-targets
+
 audit:
 	cargo deny check all
 
-migrate:
+migrate: migrate-postgres
+
+migrate-postgres:
 	cargo sqlx migrate run
 
+migrate-sqlite:
+	cargo sqlx migrate run --source migrations/sqlite
+
 build:
 	cargo build
 
@@ -27,7 +45,7 @@ format:
 format-check:
 	cargo fmt --check --all
 
-check-all: format-check lint audit test
+check-all: format-check lint audit test test-sqlite
 
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