fix(content): add price_tier to brand layer schema, import, and V-B06 (#828 blocker)

price_tier was documented in D-189 §5 and present in brands.toml but
silently discarded on import — absent from schema, INSERT, and V-B06.

- Add price_tier TEXT column to brand_products CREATE TABLE
- Add COLUMN_MIGRATIONS entry for ALTER TABLE on existing DBs
- Add VALID_PRICE_TIERS constant (mass/premium/luxury/flagship/institutional)
- Include price_tier in product_rows tuple and INSERT OR REPLACE
- Add price_tier to V-B06 enum checks; skip NULL (nullable column)
- Backfill 4 pre-amendment anchor brands (8 entries): Calloway flagship/premium,
  VGV luxury/premium, thrds luxury/premium, Bífröst flagship/luxury

V-B01..V-B06 all pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tooling/economy-db/import_economics.py

@@ -72,6 +72,7 @@ CREATE TABLE IF NOT EXISTS brand_products (
     shadow_viable INTEGER NOT NULL DEFAULT 0,
     brand_tier TEXT NOT NULL,
     halo_brand_id TEXT REFERENCES brand_products(brand_product_id),
+    price_tier TEXT,
     updated_at TEXT DEFAULT (datetime('now'))
 );
 
@@ -177,6 +178,7 @@ COLUMN_MIGRATIONS = [
     ("corporations", "behavioral_archetype", "TEXT"),
     ("corporations", "supply_chain_role", "TEXT"),
     ("corporations", "shadow_economy_access", "INTEGER DEFAULT 0"),
+    ("brand_products", "price_tier", "TEXT"),
 ]
 
 
@@ -743,6 +745,7 @@ VALID_VALUE_TRAJECTORIES = {"appreciating", "depreciating", "timeless"}
 VALID_SCARCITY_CLASSES = {"capped", "constrained", "scalable", "unlimited"}
 VALID_BRAND_TIERS = {"halo", "volume"}
 VALID_CURRENCY_DENOMINATIONS = {"tractus", "mark", "mixed", "sol_adjacent"}
+VALID_PRICE_TIERS = {"mass", "premium", "luxury", "flagship", "institutional"}
 
 
 def import_brands(
@@ -780,6 +783,7 @@ def import_brands(
             int(p.get("shadow_viable", False)),
             p["brand_tier"],
             p.get("halo_brand_id"),
+            p.get("price_tier"),
         ))
 
     input_rows = []
@@ -797,8 +801,8 @@ def import_brands(
                 value_trajectory, scarcity_class, product_subcategory,
                 base_premium_multiplier, premium_floor, origin_system,
                 terroir_locked, currency_denomination, shadow_viable,
-                brand_tier, halo_brand_id
-            ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)""",
+                brand_tier, halo_brand_id, price_tier
+            ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)""",
             product_rows,
         )
         conn.executemany(
@@ -947,12 +951,15 @@ def validate_brands(conn: sqlite3.Connection) -> list[str]:
         ("scarcity_class", VALID_SCARCITY_CLASSES),
         ("brand_tier", VALID_BRAND_TIERS),
         ("currency_denomination", VALID_CURRENCY_DENOMINATIONS),
+        ("price_tier", VALID_PRICE_TIERS),
     ]
     for column, valid_set in enum_checks:
         bad = conn.execute(
             f"SELECT brand_product_id, {column} FROM brand_products"
         ).fetchall()
         for pid, value in bad:
+            if value is None:
+                continue  # nullable columns (e.g. price_tier) may be unset
             if value not in valid_set:
                 errors.append(
                     f"V-B06: brand_product '{pid}' has {column}='{value}' — "