store: collapse migrations into a single schema.sql, drop legacy-owner path

Pre-production app, no need to preserve incremental migration history:
replace the 26 migration files with one current-state schema.sql (the
schema.sql comments are now the living documentation), simplify db.go to
apply it once instead of tracking/rebuilding through schema_migrations,
and make user_id NOT NULL everywhere now that there's no staged migration
to accommodate a nullable backfill window.

This removes the reason ClaimLegacyOwner/GENIUSRUN_LEGACY_OWNER_OIDC_SUB
existed (binding a pre-existing singleton-schema database to one account
across a staged migration), so that whole path is gone too -- the
existing dev database was wiped and reseeded fresh under the new schema.

Add cmd/dumpschema, which regenerates docs/DATABASE.md straight from the
live schema (via store.Open + sqlite_master introspection) so the
database documentation can never drift out of sync with reality.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/cmd/dumpschema/main.go

@@ -0,0 +1,101 @@
+// Command dumpschema regenerates docs/DATABASE.md from the real, live
+// database schema -- it opens a fresh temp database through store.Open
+// (the exact same code path geniusrund itself uses) and introspects
+// sqlite_master, so the generated documentation can never drift from what
+// the app actually creates. Run it after any change to
+// internal/store/schema.sql:
+//
+//	go run ./cmd/dumpschema
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"geniusrun/backend/internal/store"
+)
+
+type schemaEntry struct {
+	typ     string // "table" or "view"
+	name    string
+	tblName string
+	sql     string
+}
+
+func main() {
+	tmpDir, err := os.MkdirTemp("", "geniusrun-dumpschema")
+	must(err)
+	defer os.RemoveAll(tmpDir)
+
+	db, err := store.Open(filepath.Join(tmpDir, "schema-check.db"))
+	must(err)
+	defer db.Close()
+
+	rows, err := db.Query(`
+		SELECT type, name, tbl_name, sql FROM sqlite_master
+		WHERE sql IS NOT NULL AND name NOT LIKE 'sqlite_%'
+		ORDER BY rowid`)
+	must(err)
+	defer rows.Close()
+
+	var tables, views []schemaEntry
+	indexesByTable := map[string][]schemaEntry{}
+
+	for rows.Next() {
+		var e schemaEntry
+		must(rows.Scan(&e.typ, &e.name, &e.tblName, &e.sql))
+		switch e.typ {
+		case "table":
+			tables = append(tables, e)
+		case "view":
+			views = append(views, e)
+		case "index":
+			indexesByTable[e.tblName] = append(indexesByTable[e.tblName], e)
+		}
+	}
+	must(rows.Err())
+
+	var b strings.Builder
+	b.WriteString("# geniusrun database schema\n\n")
+	b.WriteString("Generated from the live schema via `go run ./cmd/dumpschema` -- do not hand-edit. ")
+	b.WriteString("The source of truth is `backend/internal/store/schema.sql`; regenerate this file after changing it.\n\n")
+
+	b.WriteString("## Tables\n\n")
+	for _, t := range tables {
+		fmt.Fprintf(&b, "- [`%s`](#%s)\n", t.name, t.name)
+	}
+	b.WriteString("\n")
+
+	for _, t := range tables {
+		fmt.Fprintf(&b, "## `%s`\n\n", t.name)
+		fmt.Fprintf(&b, "```sql\n%s;\n```\n\n", t.sql)
+		if idxs := indexesByTable[t.name]; len(idxs) > 0 {
+			b.WriteString("Indexes:\n\n```sql\n")
+			for _, idx := range idxs {
+				fmt.Fprintf(&b, "%s;\n", idx.sql)
+			}
+			b.WriteString("```\n\n")
+		}
+	}
+
+	if len(views) > 0 {
+		b.WriteString("## Views\n\n")
+		for _, v := range views {
+			fmt.Fprintf(&b, "### `%s`\n\n", v.name)
+			fmt.Fprintf(&b, "```sql\n%s;\n```\n\n", v.sql)
+		}
+	}
+
+	outPath := "../docs/DATABASE.md"
+	must(os.WriteFile(outPath, []byte(b.String()), 0644))
+	log.Printf("wrote %s", outPath)
+}
+
+func must(err error) {
+	if err != nil {
+		log.Fatal(err)
+	}
+}