feat(email): send booking reminders before event start.

Job process_reminders in subscription_worker; REMINDER_LEAD_HOURS window;
CalenTIQ email + in-app event_reminder; idempotent reminder_sent.
Refs EventHub/EventHubBack#70

src/logic/logic_email.erl

@@ -1,5 +1,6 @@
 -module(logic_email).
--export([send_verification_email/2, send_specialist_invite/2, send_password_reset/2]).
+-export([send_verification_email/2, send_specialist_invite/2, send_password_reset/2,
+         send_booking_reminder/4]).
 
 %% SMTP via env (variant A / Back#68):
 %%   SMTP_HOST empty → log-only (local/tests)
@@ -19,6 +20,15 @@ send_specialist_invite(Email, Token) ->
 send_password_reset(Email, Token) ->
   send_kind(reset, Email, abs_url(<<"/reset-password?token=">>, Token)).
 
+%% Reminder before event start (Back#70).
+-spec send_booking_reminder(binary(), binary(), calendar:datetime(), binary()) -> ok.
+send_booking_reminder(Email, EventTitle, StartTime, EventPath) ->
+  Url = abs_path(EventPath),
+  WhenText = format_utc(StartTime),
+  {Subject, Plain, Html} =
+    logic_email_templates:render_reminder(Url, to_bin(EventTitle), WhenText),
+  send(to_bin(Email), Subject, Plain, Html).
+
 send_kind(Kind, To0, Url) ->
   {Subject, Plain, Html} = logic_email_templates:render(Kind, Url),
   send(to_bin(To0), Subject, Plain, Html).
@@ -99,6 +109,18 @@ abs_url(PathPrefix, Token) ->
   TokenB = to_bin(Token),
   <<Base/binary, PathPrefix/binary, TokenB/binary>>.
 
+abs_path(Path0) ->
+  Base = strip_slash(env_bin("PUBLIC_APP_URL", <<"https://stage.calentiq.com">>)),
+  Path = case to_bin(Path0) of
+    <<$/, Rest/binary>> -> <<$/, Rest/binary>>;
+    Rest -> <<$/, Rest/binary>>
+  end,
+  <<Base/binary, Path/binary>>.
+
+format_utc({{Y, Mo, D}, {H, Mi, _S}}) ->
+  iolist_to_binary(io_lib:format("~4..0B-~2..0B-~2..0B ~2..0B:~2..0B",
+                                 [Y, Mo, D, H, Mi])).
+
 strip_slash(Url) ->
   case binary:last(Url) of
     $/ -> binary:part(Url, 0, byte_size(Url) - 1);