feat(affiliate): add affiliate_programs table + migration 0027

Creates affiliate_programs for centralised retailer config (URL template,
tracking tag, commission %). Adds nullable program_id + product_identifier
to affiliate_products for backward compat. Seeds "Amazon" program with
oneLink template. Backfills existing products by extracting ASINs from
baked affiliate_url values.

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

web/src/padelnomics/migrations/versions/0027_affiliate_programs.py

@@ -0,0 +1,79 @@
+"""Migration 0027: Affiliate programs table + program FK on products.
+
+affiliate_programs: centralises retailer configs (URL template + tag + commission).
+  - url_template uses {product_id} and {tag} placeholders, assembled at redirect time.
+  - tracking_tag: e.g. "padelnomics-21" — changing it propagates to all products instantly.
+  - commission_pct: stored as a decimal (0.03 = 3%) for revenue estimates.
+  - status: active/inactive — only active programs appear in the product form dropdown.
+  - notes: internal field for login URLs, account IDs, etc.
+
+affiliate_products changes:
+  - program_id (nullable FK): new products use a program; existing products keep their
+    baked affiliate_url (backward compat via build_affiliate_url() fallback).
+  - product_identifier: ASIN, product path, or other program-specific ID (e.g. B0XXXXX).
+
+Amazon OneLink note: we use a single "Amazon" program pointing to amazon.de.
+Amazon OneLink (configured in the Associates dashboard, no code changes needed)
+auto-redirects visitors to their local marketplace (UK→amazon.co.uk, ES→amazon.es)
+with the correct regional tag. One program covers all Amazon marketplaces.
+"""
+import re
+
+
+def up(conn) -> None:
+    conn.execute("""
+        CREATE TABLE affiliate_programs (
+            id              INTEGER PRIMARY KEY AUTOINCREMENT,
+            name            TEXT NOT NULL,
+            slug            TEXT NOT NULL UNIQUE,
+            url_template    TEXT NOT NULL,
+            tracking_tag    TEXT NOT NULL DEFAULT '',
+            commission_pct  REAL NOT NULL DEFAULT 0,
+            homepage_url    TEXT NOT NULL DEFAULT '',
+            status          TEXT NOT NULL DEFAULT 'active',
+            notes           TEXT NOT NULL DEFAULT '',
+            created_at      TEXT NOT NULL DEFAULT (datetime('now')),
+            updated_at      TEXT
+        )
+    """)
+
+    # Seed the default Amazon program.
+    # OneLink handles geo-redirect to local marketplaces — no per-country programs needed.
+    conn.execute("""
+        INSERT INTO affiliate_programs (name, slug, url_template, tracking_tag, commission_pct, homepage_url)
+        VALUES ('Amazon', 'amazon', 'https://www.amazon.de/dp/{product_id}?tag={tag}', 'padelnomics-21', 3.0, 'https://www.amazon.de')
+    """)
+
+    # Add program FK + product identifier to products table.
+    # program_id is nullable — existing rows keep their baked affiliate_url.
+    conn.execute("""
+        ALTER TABLE affiliate_products
+        ADD COLUMN program_id INTEGER REFERENCES affiliate_programs(id)
+    """)
+    conn.execute("""
+        ALTER TABLE affiliate_products
+        ADD COLUMN product_identifier TEXT NOT NULL DEFAULT ''
+    """)
+
+    # Backfill: extract ASIN from existing Amazon affiliate URLs.
+    # Pattern: /dp/<ASIN> where ASIN is 10 uppercase alphanumeric chars.
+    amazon_program = conn.execute(
+        "SELECT id FROM affiliate_programs WHERE slug = 'amazon'"
+    ).fetchone()
+    assert amazon_program is not None, "Amazon program must exist after seed"
+    amazon_id = amazon_program[0]
+
+    rows = conn.execute(
+        "SELECT id, affiliate_url FROM affiliate_products"
+    ).fetchall()
+    asin_re = re.compile(r"/dp/([A-Z0-9]{10})")
+    for product_id, url in rows:
+        if not url:
+            continue
+        m = asin_re.search(url)
+        if m:
+            asin = m.group(1)
+            conn.execute(
+                "UPDATE affiliate_products SET program_id=?, product_identifier=? WHERE id=?",
+                (amazon_id, asin, product_id),
+            )