Fix interactive prompt: keep reverse-video escape inside readline non-printing markers

[tkossak]

Description

In the interactive prompt, after typing characters and pressing Backspace repeatedly, the first typed character cannot be deleted (the cursor/redisplay is off by one column).

The colored prompt brackets its ANSI escapes with readline's non-printing markers \001 (RL_PROMPT_START_IGNORE) and \002 (RL_PROMPT_END_IGNORE) so they aren't counted toward the prompt width. But in the reverse-video open sequence, \002 is placed before the terminating m of \x1b[7m, leaving the m outside the ignore region:

# before
PROMPTMSG = '\001\x1b[7\002mbuku (? for help)\001\x1b[0m\002 '
#                    ^^^^ \002 closes before the terminating `m`

The terminal still consumes \x1b[7m as an invisible SGR toggle, but readline counts the leftover m as one visible column. So readline computes the prompt as 19 columns wide when it renders as 18 — an off-by-one that shifts readline's idea of where input begins, so Backspace stops one column early. The reset code \001\x1b[0m\002 is already wrapped correctly, which is why the error is exactly one column.

Fix — move \002 to after the m, so the whole \x1b[7m is inside the markers:

# after
PROMPTMSG = '\001\x1b[7m\002buku (? for help)\001\x1b[0m\002 '

Two occurrences are affected: PROMPTMSG (main reverse-video prompt) and the per-DB prompt suffix ([{bdb.dbname}]).

Design notes

Purely a correction of the readline non-printing markers around the reverse-video SGR opener; no logic, API, or option changes. The closing \x1b[0m was already correctly bracketed and is left unchanged.

Side effects

None expected. The change is inert when color is disabled (--nc / NO_COLOR), since that path uses the plain, escape-free PROMPTMSG. Not OS-specific — it's a readline redisplay issue reproducible wherever GNU readline is used.

Test cases

Minimal repro (no buku needed) — before the fix, type a word then Backspace to the start; the first char can't be removed:

import readline  # noqa: F401
prompt = '\001\x1b[7\002mbuku (? for help)\001\x1b[0m\002 '
input(prompt)

Manual verification with buku:

buku              # type "hello", hold Backspace -> clears fully, incl. first char
buku --db testdb  # exercises the [testdb] reverse-video suffix; backspace behaves too

Linting (per PR guidelines) — both clean on the change:

python3 -m flake8 buku.py
python3 -m pylint buku.py --rc-file tests/.pylintrc