Address /simplify findings on get_peaks

The OGC peaks queryables endpoint declares peak_since as `integer`,
but the wrapper typed it `str | list[str]` and the docstring offered
a fabricated string example ("site establishment"). Verified against
the live queryables: peak_since, water_year, year, month, day are
all integer-typed.

Re-typed peak_since to int | list[int] | None and rewrote the
docstring entry to describe the actual semantics (the year since
which the peak has stood as a record).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/waterdata/api.py

@@ -1483,7 +1483,7 @@ def get_peaks(
     year: int | list[int] | None = None,
     month: int | list[int] | None = None,
     day: int | list[int] | None = None,
-    peak_since: str | list[str] | None = None,
+    peak_since: int | list[int] | None = None,
     properties: str | list[str] | None = None,
     skip_geometry: bool | None = None,
     bbox: list[float] | None = None,
@@ -1526,9 +1526,10 @@ def get_peaks(
     water_year, year, month, day : int or list of ints, optional
         Calendar / water-year filters on the peak event. The water year ends
         September 30 (e.g. WY2024 = Oct 1, 2023 – Sep 30, 2024).
-    peak_since : string or list of strings, optional
-        Server-side filter for "first time the value has been exceeded since"
-        flag (e.g. ``"1900"``, ``"site establishment"``).
+    peak_since : int or list of ints, optional
+        Filter on the year since which the peak value has stood as the
+        record (the API serves this field as an integer; many rows are
+        ``null``).
     properties : string or list of strings, optional
         Subset of columns to return. Defaults to every available property.
     skip_geometry : boolean, optional