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).

Cargo.toml

@@ -18,7 +18,7 @@ categories = ["database"]
 [package]
 name = "georm"
 readme = "README.md"
-description = "Georm, a simple, opiniated SQLx ORM for PostgreSQL"
+description = "Georm, a simple, opiniated SQLx ORM for PostgreSQL and SQLite"
 authors.workspace = true
 edition.workspace = true
 homepage.workspace = true
@@ -26,13 +26,18 @@ license.workspace = true
 repository.workspace = true
 version.workspace = true
 
+[features]
+default = ["postgres"]
+postgres = ["georm-macros/postgres", "sqlx/postgres", "sqlx/bigdecimal"]
+sqlite = ["georm-macros/sqlite", "sqlx/sqlite"]
+
 [workspace.dependencies]
 georm-macros = { version = "=0.2.1", path = "georm-macros" }
 
 [workspace.dependencies.sqlx]
 version = "0.8.6"
 default-features = false
-features = ["postgres", "runtime-tokio", "macros", "migrate", "bigdecimal"]
+features = ["runtime-tokio", "macros", "migrate"]
 
 [dependencies]
 sqlx = { workspace = true }
@@ -43,9 +48,28 @@ chrono = { version = "0.4", features = ["serde"] }
 rand = "0.9"
 
 [dev-dependencies.sqlx]
+# Unlike the crate's own [features] above, dev-dependencies are always built
+# for `cargo test` regardless of --no-default-features/--features passed for
+# the georm package itself. Tests bind directly to sqlx::PgPool/SqlitePool,
+# so both drivers must be enabled here unconditionally.
 version = "0.8.6"
 default-features = false
-features = ["postgres", "runtime-tokio", "macros", "migrate", "chrono"]
+features = ["postgres", "sqlite", "runtime-tokio", "macros", "migrate", "chrono", "bigdecimal"]
+
+# Cargo's test auto-discovery only scans tests/*.rs (not subdirectories), so
+# the tests/sqlite/*.rs files (kept in their own directory to avoid colliding
+# with the postgres tests/models.rs module) need explicit [[test]] entries.
+[[test]]
+name = "sqlite_simple_struct"
+path = "tests/sqlite/simple_struct.rs"
+
+[[test]]
+name = "sqlite_defaultable_struct"
+path = "tests/sqlite/defaultable_struct.rs"
+
+# sqlite_composite_key intentionally deferred: UserRole's chrono::DateTime<Utc>
+# column needs per-field query_as! type overrides under SQLite (see the note
+# in tests/sqlite/models.rs) that don't exist yet.
 
 [workspace.lints.rust]
 unsafe_code = "forbid"