feat: config model and yaml loader

Add Pydantic config models (SqlServerConfig, AccessConfig, RuntimeConfig,
FileMapping, SyncConfig) and a YAML loader (load_config). FileMapping provides
source_path() and target_table() helpers; number-typed YAML keys/values (e.g.
root: 2026) are coerced to str via coerce_numbers_to_str. Includes
config.example.yaml template (config.yaml with real credentials stays
gitignored) and pyproject.toml pytest config (pythonpath=src).

Co-Authored-By: Claude <noreply@anthropic.com>

src/sync/config.py

@@ -0,0 +1,50 @@
+from __future__ import annotations
+from pathlib import Path
+import yaml
+from pydantic import BaseModel, ConfigDict, Field
+
+class SqlServerConfig(BaseModel):
+    conn_str: str
+    sync_queue_table: str = "dbo.SyncQueue"
+
+class AccessConfig(BaseModel):
+    model_config = ConfigDict(coerce_numbers_to_str=True)
+    driver: str
+    roots: dict[str, str]
+
+class RuntimeConfig(BaseModel):
+    poll_interval_seconds: int = 10
+    capture_batch_size: int = 500
+    apply_batch_size: int = 200
+    max_retries: int = 5
+    retry_backoff_seconds: int = 30
+    cleanup_batch_size: int = 200
+    cleanup_lock_retries: int = 3
+
+class FileMapping(BaseModel):
+    model_config = ConfigDict(coerce_numbers_to_str=True)
+    file: str
+    root: str
+    schema: str
+    year_suffix: str = ""
+    exclude_tables: list[str] = Field(default_factory=list)
+    include_tables: list[str] | None = None
+
+    def source_path(self, cfg: "SyncConfig") -> str:
+        base = cfg.access.roots[self.root]
+        return f"{base}\\{self.file}"
+
+    def target_table(self, access_table: str) -> str:
+        return f"{access_table}{self.year_suffix}"
+
+class SyncConfig(BaseModel):
+    sql_server: SqlServerConfig
+    access: AccessConfig
+    runtime: RuntimeConfig
+    files: list[FileMapping]
+    logging: dict | None = None
+
+def load_config(path: str) -> SyncConfig:
+    with open(path, "r", encoding="utf-8") as f:
+        data = yaml.safe_load(f)
+    return SyncConfig(**data)