Adds a pluggable database configuration backend

[rbarreiros]

Sorry, this is an extensive patch!

Introduces a pluggable backend system for AsyncConfig that allows SVXLink configuration to be stored in a relational database (SQLite, MySQL/MariaDB, or PostgreSQL) in addition to the traditional .conf file, with optional live change notifications and hot-reload support. This backend is extendable and allows for other backends to be implemented following the same guidelines as Async*ConfigBackend classes.

Please read src/doc/README_DBCONFIG.md for more information and examples.

New library components (src/async/core/):

• AsyncConfigBackend: abstract base class for all backends, with a factory registry, table-prefix support, and a non-blocking polling infrastructure (background thread + self-pipe + FdWatch) so that slow DB queries never stall the event loop.

• AsyncFileConfigBackend: wraps the existing file parser as a proper backend plugin, preserving full backward compatibility.

• AsyncSQLiteConfigBackend: single-file embedded database backend (optional, enabled when libsqlite3 is detected at build time).

• AsyncMySQLConfigBackend: MySQL/MariaDB backend (optional, enabled when libmysqlclient or libmariadb is detected at build time). Connection string format: key=value pairs separated by semicolons.

• AsyncPostgreSQLConfigBackend: PostgreSQL backend (optional, enabled when libpq is detected at build time). Connection string is passed directly to PQconnectdb() as libpq keyword=value pairs.

• AsyncConfigSource: URL parser and backend-availability registry used by the factory functions createConfigBackend() and createConfigBackendByType().

AsyncConfig rework (src/async/core/AsyncConfig.*):

• Replaced the monolithic open(filename) call with a clean fallback chain implemented in openWithFallback(): --config file → --dbconfig db.conf → auto-discover db.conf in standard paths → auto-discover application .conf file → fatal error. This change shifts to AsyncConfig all the config (file or other) reading logic instead of the multiple code repetition in each binary.

• Added openFromDbConfig(path) and openDirect(url) as lower-level entry points for callers that bypass the fallback chain.

• Centralized CFG_DIR processing into Config::loadCfgDir(); removed duplicate code from every application main().

• Added valueUpdated sigc++ signal, emitted whenever any config value changes (from setValue, backend poll, or SIGHUP reload).

• Added subscribeValue() templates that register a typed callback invoked immediately with the current value and again on every future change. Returns a SubId handle.

• Added subscribeOptionalValue() for keys that must not be auto-created when absent.

• Added unsubscribeValue(section, tag, SubId) to safely remove a registered callback, preventing dangling-lambda crashes when a subscriber is destroyed before the Config object.

• Fixed reload() to correctly handle sections and tags that were added to the database after startup (previously new keys were silently ignored).

• Added importFromConfigFile(filename) as the public entry point for seeding an empty database from an installed .conf file.

• Added getLastError(), getMainConfigFile(), getBackendType() for better diagnostics in application main().

Application changes (all binaries):

• Added --dbconfig option to specify a db.conf path explicitly, overriding the automatic search.

• Added --init-db option to import the installed example .conf file (e.g. svxlink.conf) into an empty database backend, then exit. Import is skipped with a warning if the database already has rows.

• Removed per-application CFG_DIR processing loops and hardcoded database seeding blocks; replaced with calls to the centralized AsyncConfig methods.

Runtime hot-reload (config value changes without restart):

• Logic, RepeaterLogic, ReflectorLogic, ReflectorV2Logic: connect to valueUpdated and implement cfgUpdated() for parameters that can be changed on-the-fly (callsign, RF thresholds, IDs, timeouts, etc.).

• Module base class: cfgUpdated() virtual method propagates valueUpdated events to all loaded modules. ModuleDtmfRepeater, ModuleEchoLink, ModuleFrn, ModuleMetarInfo, ModuleParrot, ModuleTrx: override cfgUpdated() for their own hot-updatable parameters.

• Squelch, LocalRxBase: cfgUpdated() for squelch thresholds and related RF parameters.

• QsoRecorder: cfgUpdated() for recording parameters.

• RfUplink, NetUplink, NetTrxAdapter: cfgUpdated() for remote TRX and network parameters.

• SipLogic, QsoFrn: cfgUpdated() support for SIP and FRN parameters.

• Voter, DtmfDecoder: subscribeValue() for voting and DTMF parameters.

• Logic.tcl: new proc config_updated {section tag value} called by the C++ cfgUpdated() handler so TCL event scripts can react to runtime configuration changes.

Build:

• CMakeLists.txt: optional detection of libsqlite3, libmysqlclient / libmariadb, and libpq; each backend is compiled in only when its development library is found. Defines HAS_SQLITE_SUPPORT, HAS_MYSQL_SUPPORT, HAS_POSTGRESQL_SUPPORT accordingly.

• Added -D_REENTRANT for the background polling thread.

New example files:

• examples/db.conf: fully-commented template for the database config file, placed in the standard search paths.

• examples/db.conf.sqlite, db.conf.mysql, db.conf.postgresql, db.conf.file: per-backend example files.

New tests added for async config backend (requires catch2)

• added some test cases to check async config backend, they can be enabled by passing -DBUILD_TESTS=ON to cmake.

• make all will build tests if enabled, make test will run ctest, running all tests.

• Single test can be compiled and run,
  make AsyncConfigBackend_test
  ./tests/async/core/AsyncConfigBackend_test

• A specific backend can be tested
  ./tests/async/core/AsyncConfigBackend_test "[sqlite]"
  ./tests/async/core/AsyncConfigBackend_test "[mysql]"
  ./tests/async/core/AsyncConfigBackend_test "[postgres]"
  ./tests/async/core/AsyncConfigBackend_test "[file]"

Tests which require a database like mysql and postgresql require a connection string.

SVXLINK_TEST_MYSQL_CONN 
SVXLINK_TEST_POSTGRESQL_CONN

example: 
export SVXLINK_TEST_POSTGRESQL_CONN="host=localhost port=5432 dbname=svxlink user=svxlink password=svxlink" 
export SVXLINK_TEST_MYSQL_CONN="host=localhost;port=3306;user=svxlink;password=svxlink;database=svxlink"

[rbarreiros]

I forgot I'm using std::optional and didn't specify the c++17 in cmake. Do you wish me to remove std::optional or add c++17 requirement ?

P.S. I forgot the structured bindings as well, I think the old debian buster already supports c++17 :)

[rbarreiros]

So sorry for these late commits Tobias... Pretty tiring few days and was in a hurry to get the PR so I missed a few important things in a refactoring I did a while ago. I made a quick test on all backends, including polling and the thread handling when the backend goes away and everything seems ok now.