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>

.gitignore

@@ -5,6 +5,10 @@ __pycache__/
 .env.local
 .env.*.local
 
+# Dynaconf (ADR-024)
+.secrets.toml
+settings.local.toml
+
 # Git
 worktrees/