feat: Add API endpoint for listing oneliners

This commit introduces a new API endpoint `/api/oneliners` that allows
authenticated users to retrieve a list of their recent oneliners.

The endpoint supports conditional GET requests using the `ETag` header
for efficient caching. It also allows specifying a time window for the
oneliners via the `hours` query parameter, with sensible defaults and
clamping.

The implementation includes:
- A new route handler `handle_oneliners`.
- A new type `OnelinerWatermark` for tracking user-specific timestamps.
- Logic for scanning user data directories, filtering by time, and
  handling deleted cases.
- Test cases to cover various scenarios, including caching, time
  windowing, and edge cases.

Cargo.lock

@@ -1187,6 +1187,7 @@ dependencies = [
  "bcrypt",
  "doctate-common",
  "dotenvy",
+ "filetime",
  "pulldown-cmark",
  "rand 0.8.6",
  "reqwest",
@@ -1488,6 +1489,17 @@ dependencies = [
  "simd-adler32",
 ]
 
+[[package]]
+name = "filetime"
+version = "0.2.27"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f98844151eee8917efc50bd9e8318cb963ae8b297431495d3f758616ea5c57db"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "libredox",
+]
+
 [[package]]
 name = "find-msvc-tools"
 version = "0.1.9"