fix(deps): align contradictory psutil/Flask-Limiter/freetype-py pins

requirements.txt's optional-install comment recommended psutil>=5.9,<6.0
while web_interface/requirements.txt hard-requires >=6.0,<7.0 — anyone
following the comment ends up with an unsatisfiable pair. The comment now
recommends the same range the web interface requires (all psutil APIs
used — Process, boot_time, cpu_percent, disk_usage, virtual_memory — are
stable in 6.x). Flask-Limiter gains the same <4.0 cap in both files and
freetype-py the same >=2.5.1 floor.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

requirements.txt

@@ -59,7 +59,9 @@ mypy>=1.5.0,<2.0.0
 # psutil            — per-plugin resource monitoring in
 #                     src/plugin_system/resource_monitor.py. The monitor
 #                     silently no-ops when missing (PSUTIL_AVAILABLE = False).
-#   pip install 'psutil>=5.9.0,<6.0.0'
+#                     Note: web_interface/requirements.txt requires this
+#                     range as a hard dependency — keep the two in sync.
+#   pip install 'psutil>=6.0.0,<7.0.0'
 #
 # Flask-Limiter     — request rate limiting in web_interface/app.py
 #                     (accidental-abuse protection, not security). The