feat: implement dynaconf configuration management (ADR-024 phases 1-3)

Replace ~80 manual os.getenv() calls in config.py with dynaconf-backed
configuration, enabling TOML file-based config alongside existing env
var support. Zero breaking changes — Settings dataclass interface and
all consumers unchanged.

Phase 1: Create settings.toml with all config keys and defaults,
.secrets.toml.example template, update .gitignore, initialize Dynaconf
instance with envvar_prefix=False and environment section switching.

Phase 2: Wire adapter — replace os.getenv() with _dynaconf.get() in
get_settings(), get_document_processor_config(), and deprecation/
dependency resolution helpers. Automatic type coercion eliminates ~30
manual int()/float()/.lower()=="true" patterns.

Phase 3: Add 12 declarative validators for port ranges, positive
integers, enum constraints, and float ranges. Remove redundant negative
overlap check from Settings.__post_init__.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/conftest.py

@@ -0,0 +1,21 @@
+"""Unit test configuration — shared fixtures for all unit tests."""
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _reload_dynaconf_after_test():
+    """Ensure dynaconf cache is clean between tests.
+
+    Dynaconf caches env var values at load time. Tests that modify os.environ
+    must call _reload_config() to refresh the cache. This fixture reloads
+    after each test to prevent leaked state.
+
+    Uses _dynaconf.reload() directly (without validate_all) since the
+    real env may have values that don't pass validators. Tests that need
+    validation should call _reload_config() explicitly.
+    """
+    yield
+    from nextcloud_mcp_server.config import _dynaconf
+
+    _dynaconf.reload()