Refactor SQLPage functions into convention-driven modules

src/webserver/database/sqlpage_functions/README.md

@@ -0,0 +1,35 @@
+# Built-in SQL functions
+
+Each built-in `sqlpage.*` function is a plain `async fn` in its own file in
+[`functions/`](functions). The file stem is the SQL function name and must match the Rust function it
+exports:
+
+```rust
+use std::borrow::Cow;
+
+use crate::webserver::http_request_info::RequestInfo;
+
+pub(super) async fn example(request: &RequestInfo, value: Option<Cow<'_, str>>) -> Option<String> {
+    // ...
+}
+```
+
+To add `sqlpage.example`, create `functions/example.rs` and add it to the
+[`sqlpage_functions!`](function_traits.rs) call in [`functions.rs`](functions.rs):
+
+```rust
+sqlpage_functions! {
+    // ...
+    example,
+}
+```
+
+The [`sqlpage_functions!`](function_traits.rs) macro declares the modules and generates the
+`SqlPageFunctionName` enum the SQL engine dispatches on. Per-function argument extraction, dispatch,
+and return-value conversion are handled generically in [`function_traits.rs`](function_traits.rs) by
+the `Extract`, `Handler`, and `IntoCowResult` traits. A function's argument and return types are read
+straight from its signature, so supported argument types are the types that implement `Extract` there.
+Functions can take up to five arguments.
+
+Keep helpers and unit tests that are specific to a function in that function's file. Shared helpers can
+be made `pub(super)` and imported by name from sibling function modules.