refactor(aprs): drop unreachable defensive branches, force UTC since

- decode_packet_row no longer guards Decimal.to_float / DateTime.from_naive
  with passthrough clauses — Postgrex returns Decimal for numeric and
  NaiveDateTime for timestamp without time zone, so the WHERE clause
  filters out the only path that could have hit the alternate branches.
- recent_packets_with_paths shifts :since to UTC before to_naive so a
  caller passing a non-UTC %DateTime{} doesn't silently match the wrong
  window.
- moduledoc now documents that the module raises on connection/SQL
  errors and is unsafe in async contexts without a catch.

lib/microwaveprop/aprs.ex

@@ -15,6 +15,15 @@ defmodule Microwaveprop.Aprs do
   Tests run against a local `aprsme_test` database whose schema mirrors
   aprs.me's partitioned `packets` table; rows are inserted via raw SQL
   in a setup block under the `Ecto.Adapters.SQL.Sandbox`.
+
+  > #### Error semantics {: .warning}
+  >
+  > Both queries call `Ecto.Adapters.SQL.query!/3` and will raise on
+  > connection or SQL failures. Today the only caller is the
+  > `Calibrate.Aprs144` mix task, where a hard exit is the right
+  > ergonomic. Async callers (`Task.async_stream`, LiveView
+  > `start_async`, etc.) MUST catch and log per the project's
+  > swallowed-error rule in CLAUDE.md.
   """
 
   alias Ecto.Adapters.SQL
@@ -73,8 +82,12 @@ defmodule Microwaveprop.Aprs do
     since = Keyword.get_lazy(opts, :since, fn -> DateTime.add(DateTime.utc_now(), -3600, :second) end)
     limit = Keyword.get(opts, :limit, 50_000)
 
+    # Force the bound to UTC so a non-UTC %DateTime{} doesn't silently
+    # match a wrong window after `to_naive` strips the tz tag.
+    naive_since = since |> DateTime.shift_zone!("Etc/UTC") |> DateTime.to_naive()
+
     %Postgrex.Result{rows: rows} =
-      SQL.query!(AprsRepo, @recent_packets_sql, [DateTime.to_naive(since), limit])
+      SQL.query!(AprsRepo, @recent_packets_sql, [naive_since, limit])
 
     Enum.map(rows, &decode_packet_row/1)
   end
@@ -100,17 +113,14 @@ defmodule Microwaveprop.Aprs do
       id: id,
       sender: sender,
       base_callsign: base_callsign,
-      lat: to_float(lat),
-      lon: to_float(lon),
-      path: path || "",
-      received_at: to_utc_datetime(received_at)
+      lat: Decimal.to_float(lat),
+      lon: Decimal.to_float(lon),
+      path: path,
+      received_at: DateTime.from_naive!(received_at, "Etc/UTC")
     }
   end
 
   defp to_float(%Decimal{} = d), do: Decimal.to_float(d)
-  defp to_float(n) when is_float(n), do: n
-  defp to_float(n) when is_integer(n), do: n * 1.0
 
   defp to_utc_datetime(%NaiveDateTime{} = naive), do: DateTime.from_naive!(naive, "Etc/UTC")
-  defp to_utc_datetime(%DateTime{} = dt), do: dt
 end