ychat: add Mode B - embedded SQLite backend for real user accounts (task cs0)

Adds a second database backend alongside the (already-broken/dormant) MySQL
one: --enable-sqlite (configure.ac/configure, mirrors --enable-mysql),
USE_SQLITE (glob.h, mirrors USE_MYSQL), and a con/data implementation using
sqlite3_prepare_v2/bind/step (parameterized queries - safer than the MySQL
path's hand-rolled character-transliteration escaping). chat.database.dbname
doubles as the SQLite file path; the "user" table is created on first
connect (CREATE TABLE IF NOT EXISTS) since SQLite has no separate schema-
provisioning step. New Dockerfile.sqlite (Mode B) builds and runs this in
Rocky Linux 9; DOCKER-SQLITE.md documents everything in detail.

DATABASE had never actually been compiled before this (Mode A always
disables it, and --enable-mysql doesn't work - configure.ac registers it as
AC_ARG_ENABLE(mysqlclient,...) but the gating check tests a third, never-set
$enable_mysql - left alone, this task is about moving away from MySQL, not
fixing it). Getting DATABASE to compile and actually run for the first time
surfaced two real, previously-undetectable bugs, both fixed:

- class data collided with std::data() (C++17) under "using namespace std"
  ("reference to 'data' is ambiguous") - renamed to ychatdb throughout
  (data.h/cpp, wrap.h/cpp, yc_register.cpp). Same bug class as the
  function->mod_func_t rename already made in glob.h.
- data_base.cpp's config-query parser used "unsigned i_pos" for a
  string::npos comparison - truncating npos to 32-bit makes the
  "no more tokens" check never true, and i_pos+1 wraps back to 0, so the
  loop never advances: an infinite loop that OOM-killed the container within
  seconds of startup. Fixed to size_t (same bug class already fixed
  repeatedly in ../yhttpd).

Verified in Docker: register creates a row, wrong password is rejected,
correct password succeeds, and - the actual point of this task - a second
user's login still works identically after a full container restart with
the db file on a bind-mounted volume, proving persistence. Independently
reproduced by a fresh-context review agent, which also rebuilt +
re-verified the whole flow itself.

Not deployed to f3s - this is a local proof of concept alongside the live
Mode A (in-memory guest chat) deployment.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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