feat(02-01): add gnomAD constraint data models and download module

- Create evidence layer package structure
- Define ConstraintRecord Pydantic model with NULL preservation
- Implement streaming download with httpx and tenacity retry
- Add lazy TSV parser with column name variant handling
- Add httpx and structlog dependencies

src/usher_pipeline/evidence/gnomad/models.py

@@ -0,0 +1,58 @@
+"""Data models for gnomAD constraint metrics."""
+
+from pydantic import BaseModel
+
+# gnomAD v4.1 constraint metrics download URL
+GNOMAD_CONSTRAINT_URL = (
+    "https://storage.googleapis.com/gcp-public-data--gnomad/release/4.1/constraint/"
+    "gnomad.v4.1.constraint_metrics.tsv"
+)
+
+# Column name mapping for different gnomAD versions
+# v2.1.1 uses: gene, transcript, pLI, oe_lof_upper (LOEUF), mean_proportion_covered_bases
+# v4.x uses: gene, transcript, mane_select, lof.pLI, lof.oe_ci.upper, mean_proportion_covered
+COLUMN_VARIANTS = {
+    "gene_id": ["gene", "gene_id"],
+    "gene_symbol": ["gene_symbol", "gene"],
+    "transcript": ["transcript", "canonical_transcript", "mane_select"],
+    "pli": ["pLI", "lof.pLI", "pli"],
+    "loeuf": ["oe_lof_upper", "lof.oe_ci.upper", "oe_lof", "loeuf"],
+    "loeuf_upper": ["oe_lof_upper_ci", "lof.oe_ci.upper", "oe_lof_upper"],
+    "mean_depth": ["mean_coverage", "mean_depth", "mean_cov"],
+    "cds_covered_pct": [
+        "mean_proportion_covered_bases",
+        "mean_proportion_covered",
+        "cds_covered_pct",
+    ],
+}
+
+
+class ConstraintRecord(BaseModel):
+    """gnomAD constraint metrics for a single gene.
+
+    Attributes:
+        gene_id: Ensembl gene ID (e.g., ENSG00000...)
+        gene_symbol: HGNC gene symbol
+        transcript: Canonical transcript ID
+        pli: Probability of being loss-of-function intolerant (NULL if no estimate)
+        loeuf: Loss-of-function observed/expected upper bound fraction (NULL if no estimate)
+        loeuf_upper: Upper bound of LOEUF confidence interval
+        mean_depth: Mean exome sequencing depth across CDS
+        cds_covered_pct: Fraction of CDS bases with adequate coverage (0.0-1.0)
+        quality_flag: Data quality indicator - "measured", "incomplete_coverage", or "no_data"
+        loeuf_normalized: Normalized LOEUF score (0-1, inverted: higher = more constrained)
+
+    CRITICAL: NULL values represent missing data and are preserved as None.
+    Do NOT convert NULL to 0.0 - "unknown" is semantically different from "zero constraint".
+    """
+
+    gene_id: str
+    gene_symbol: str
+    transcript: str
+    pli: float | None = None
+    loeuf: float | None = None
+    loeuf_upper: float | None = None
+    mean_depth: float | None = None
+    cds_covered_pct: float | None = None
+    quality_flag: str = "no_data"
+    loeuf_normalized: float | None = None