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>

.secrets.toml.example

@@ -0,0 +1,13 @@
+# Secrets configuration for Nextcloud MCP Server.
+# Copy this file to .secrets.toml and fill in your values.
+# .secrets.toml is gitignored — never commit real secrets.
+#
+# See docs/ADR-024-dynaconf-configuration-management.md for details.
+
+[default]
+token_encryption_key = ""
+nextcloud_password = ""
+nextcloud_oidc_client_secret = ""
+qdrant_api_key = ""
+openai_api_key = ""
+custom_processor_api_key = ""