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>

tests/test_config.py

@@ -0,0 +1,44 @@
+import pathlib, textwrap
+from sync.config import load_config, FileMapping
+
+def test_load_config_parses_fields(tmp_path):
+    cfg_text = textwrap.dedent("""
+    sql_server:
+      conn_str: "Driver={ODBC Driver 17 for SQL Server};Server=s;Database=d;UID=u;PWD=p;"
+      sync_queue_table: "dbo.SyncQueue"
+    access:
+      driver: "{Microsoft Access Driver (*.accdb, *.mdb)}"
+      roots:
+        2026: "\\\\\\\\srv\\\\2026"
+        2025: "\\\\\\\\srv\\\\2025"
+    runtime:
+      poll_interval_seconds: 10
+      capture_batch_size: 500
+      apply_batch_size: 200
+      max_retries: 5
+      retry_backoff_seconds: 30
+      cleanup_batch_size: 200
+      cleanup_lock_retries: 3
+    files:
+      - file: "氩弧焊.accdb"
+        root: 2026
+        schema: "TIGWelding"
+        year_suffix: "_YEAR2026"
+        exclude_tables: ["TableChangeLog", "氩弧焊每日催货落实记录_停"]
+    """)
+    p = tmp_path / "config.yaml"
+    p.write_text(cfg_text, encoding="utf-8")
+    cfg = load_config(str(p))
+    assert cfg.sql_server.sync_queue_table == "dbo.SyncQueue"
+    assert cfg.runtime.poll_interval_seconds == 10
+    assert cfg.files[0].schema == "TIGWelding"
+    assert cfg.files[0].year_suffix == "_YEAR2026"
+    assert "2026" in cfg.files[0].source_path(cfg)
+
+def test_file_mapping_target_table_applies_year_suffix():
+    fm = FileMapping(file="x.accdb", root="2026", schema="s", year_suffix="_YEAR2026",
+                     exclude_tables=[], include_tables=None)
+    assert fm.target_table("一车间记录") == "一车间记录_YEAR2026"
+    fm2 = FileMapping(file="y.accdb", root="2025", schema="s", year_suffix="",
+                      exclude_tables=[], include_tables=None)
+    assert fm2.target_table("26年压力表合同数据") == "26年压力表合同数据"