fix(startup): warn when an installed systemd unit has drifted from the repo's

Nothing re-applies systemd units after the first install. `git pull` -- which
is what the web UI's update button runs -- brings a new template into the
checkout, but no code in web_interface/ or src/ copies it to
/etc/systemd/system, and nothing anywhere runs `systemctl daemon-reload`. The
unit that actually runs is whatever first_time_install.sh wrote on day one.

So every hardening added to a unit is inert on existing installs, silently.
Measured on a live rig:

    installed  /etc/systemd/system/ledmatrix.service   2026-08-06
    template   systemd/ledmatrix.service               2026-08-19
    contents                                           differ

with the practical result that the MemoryMax=85% the repo's template specifies
was not being enforced at all -- `systemctl show` reported
MemoryMax=infinity. Anyone reading the template would reasonably believe the
service was capped.

Startup now compares each installed unit against its substituted template and
warns when they differ, naming install_service.sh as the remedy.

A warning, not an error, and deliberately not a silent rewrite: editing files
under /etc and restarting services is the installer's job, not something a
display process should do to a machine while it is booting. Making it fatal
would also brick every development checkout whose unit is legitimately absent
or hand-edited.

Comparison ignores comments, blank lines and ordering. The template carries
explanatory comments the installed copy will not have, and systemd does not
care about order within a section, so a literal comparison would warn on every
boot and be ignored within a week.

Mutation-checked three ways: never reporting drift fails, making it fatal
fails, and -- after the first attempt missed it -- comparing raw text now fails
too. That last gap is worth noting: the comment-insensitivity tests originally
exercised the helper directly, so a comparison that stopped calling the helper
passed them all. The test that catches it goes through _validate_systemd_units.

29 startup-validator tests pass.

(cherry picked from commit cf521bdfd8)

src/startup_validator.py

@@ -62,6 +62,9 @@ class StartupValidator:
         # Validate plugins if plugin manager is available
         if self.plugin_manager:
             self._validate_plugins()
+
+        # Warn when the running systemd unit no longer matches the repo's
+        self._validate_systemd_units()
         
         is_valid = len(self.errors) == 0
         
@@ -74,6 +77,71 @@ class StartupValidator:
         
         return (is_valid, self.errors.copy(), self.warnings.copy())
     
+    #: Units this project installs, and where each is installed to.
+    _UNITS = (
+        ("systemd/ledmatrix.service", "/etc/systemd/system/ledmatrix.service"),
+        ("systemd/ledmatrix-web.service", "/etc/systemd/system/ledmatrix-web.service"),
+    )
+
+    def _validate_systemd_units(self) -> None:
+        """Warn when an installed unit has drifted from the repo's template.
+
+        Nothing re-applies these after the first install. `git pull` -- which is
+        what the web UI's update button runs -- brings a new template into the
+        checkout, but nothing copies it to /etc/systemd/system and nothing runs
+        `systemctl daemon-reload`, so the unit that actually runs is whatever
+        first_time_install.sh wrote on day one.
+
+        That makes every hardening added to a unit inert on existing installs.
+        Measured on one rig: the installed unit was thirteen days older than the
+        repo's and differed in content, so a MemoryMax the repo had specified
+        was not being enforced at all -- `systemctl show` reported
+        MemoryMax=infinity.
+
+        A warning rather than an error, and certainly not a silent rewrite:
+        editing files under /etc and restarting services is the installer's job,
+        not something a display process should do to a machine while it boots.
+        The remedy is to re-run scripts/install/install_service.sh.
+        """
+        try:
+            project_root = Path(__file__).resolve().parent.parent
+            for template_rel, installed_path in self._UNITS:
+                template = project_root / template_rel
+                installed = Path(installed_path)
+                if not template.is_file() or not installed.is_file():
+                    continue
+
+                # The template carries placeholders the installer substitutes,
+                # so compare the substituted form rather than the raw file.
+                expected = template.read_text(encoding="utf-8")
+                expected = expected.replace("__PROJECT_ROOT_DIR__", str(project_root))
+                expected = expected.replace("__USER__", "root")
+
+                try:
+                    actual = installed.read_text(encoding="utf-8")
+                except PermissionError:
+                    continue
+
+                if self._unit_body(expected) != self._unit_body(actual):
+                    self.warnings.append(
+                        f"{installed.name} differs from {template_rel}; the "
+                        "installed unit is not refreshed by an update, so "
+                        "settings added to the template are not in effect. "
+                        "Re-run scripts/install/install_service.sh to apply them."
+                    )
+        except OSError as e:
+            self.logger.debug("Could not compare systemd units: %s", e)
+
+    @staticmethod
+    def _unit_body(text: str) -> str:
+        """A unit's meaningful lines: no comments, no blanks, no ordering noise."""
+        lines = []
+        for line in text.splitlines():
+            line = line.strip()
+            if line and not line.startswith("#"):
+                lines.append(line)
+        return "\n".join(sorted(lines))
+
     def _validate_config(self) -> None:
         """Validate configuration files."""
         try: