delivery-log-storage: the table for a message's log lines

Migration 0003_send_log_lines.sql, DDL only: queue id, the line's own time,
the line. No reference to send_log, because a message's first lines are
logged before the journal milter writes its row and one queue id is several
rows.

Store methods: InsertSendLogLine; InsertSendLogLineOnce for lines the tailer
reads twice after an unclean stop; SendLogLines, which gives a row the lines
of its queue id dated from an hour before the row to the queue lifetime after
it, so a later message under a reused id is not shown; PruneSendLogLines,
which deletes lines whose queue id has no send-log row - the whole of the
table's retention. A line is cut to 4096 bytes.

Nothing writes or reads the table yet. Checked on Windows: go vet and
go test ./internal/store pass, with the schema test pinning the new table,
its index and user_version 3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/migrations/0003_send_log_lines.sql

@@ -0,0 +1,22 @@
+-- What Postfix wrote about a message, kept with its send-log rows
+-- (delivery-log-storage, 2.2.0). DDL only: a 2.1.0 database gains one empty
+-- table and nothing in it changes.
+
+-- One row per mail.log line headed by a queue id, written by the log tailer as
+-- it reads the log. mail.log is rotated daily and fourteen files are kept; a
+-- send-log row lives for the retention window, ninety days by default, and
+-- its page shows these lines for as long.
+--
+-- There is no reference to send_log: the first lines of a message are logged
+-- before the journal milter writes its row, and a message with several
+-- recipients is several rows under one queue id. A line belongs to a row by
+-- queue id and by time (Postfix reuses queue ids), and a line whose queue id
+-- has no send-log row is deleted by the tailer's prune.
+CREATE TABLE send_log_lines (
+    id        INTEGER PRIMARY KEY AUTOINCREMENT,
+    queue_id  TEXT NOT NULL,
+    logged_at TEXT NOT NULL,
+    line      TEXT NOT NULL
+);
+-- A message's page reads the lines of one queue id within a span of time.
+CREATE INDEX idx_send_log_lines_queue_id ON send_log_lines (queue_id, logged_at);