Fix broken series-ID builders; env-var config; project README

Repair every dead series-ID encoding (library now 82/82 query helpers
return live data, verified against the BLS API):

- JOLTS: 21-char format (was 18) — add state/area/sizeclass fields
- OES: national area code 0000000 (was invalid 0000400)
- ECI: correct owner/component/estimate encoding, default unadjusted (CIU)
- Productivity: 4-digit sector + 4-digit measure codes (was 2+3)
- QCEW: 13-char timeseries-API form (ENUUS00010510 / ENU{fips}00010{own}10)
- PPI: repoint finished-goods -> final demand (WPUFD4); keep alias
- ECEC: drop fabricated health-insurance/retirement helpers; add total benefits
- wages SOC: software developers 151132 -> 151252 (2018 SOC)

Tooling/docs:
- config reads BLS_API_KEY env var (takes precedence; config.py gitignored)
- add requirements.txt
- rewrite README as project front door + coverage table + limitations
- correct JOLTS/OES tables in series_id_formats.md, USAGE.md, dataset explorer

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bls_client/queries/employment.py

@@ -113,21 +113,27 @@ def jolts_dashboard() -> dict:
 # QCEW
 # ---------------------------------------------------------------------------
 def qcew_national_private() -> str:
-    """QCEW — national, private sector, all industries, quarterly."""
-    return "ENU0000010510000"
+    """QCEW — national, private sector, all industries (monthly employment)."""
+    return "ENUUS00010510"
 
 
 def qcew_dc_private() -> str:
     """QCEW — DC, private sector, all industries."""
-    return "ENU1100010510000"
+    return qcew_state(11, "5")
 
 
 def qcew_state(state_fips: int, ownership: str = "5") -> str:
     """
     QCEW state-level series.
 
+    Format: EN + U + area(5: 2-digit FIPS + "000") + datatype "1" + size "0"
+            + ownership(1) + industry "10" (total, all industries) = 13 chars.
+
     Args:
-        state_fips: 2-digit FIPS
+        state_fips: 2-digit FIPS (e.g. 11=DC)
         ownership:  "0"=all, "5"=private, "1"=federal, "2"=state, "3"=local
+
+    Note: BLS serves the full QCEW catalog (county/industry detail) through its
+    dedicated QCEW Open Data API, not the timeseries API used here.
     """
-    return f"ENU{state_fips:02d}0001{ownership}10000"
+    return f"ENU{state_fips:02d}00010{ownership}10"