docs: describe the system that exists, not the one we started with

The README still opened on "Primary School Compass", a KS2 tool for Wandsworth
and Merton served by FastAPI and vanilla JavaScript with Chart.js. Every layer
of that sentence is now wrong: coverage is England-wide across KS2, KS4,
all-through and post-16, Next.js owns the public UI, and school data comes from
dbt-built `marts.*` rather than CSVs loaded at startup. The setup instructions
walked a reader into a virtualenv and a CSV import that cannot build the current
schema, so following the docs produced an empty database and a wrong mental
model at the same time.

Replace the narrative docs with two reference documents that were checked
against the code: docs/ARCHITECTURE.md for request flow, data ownership, the
backend/frontend module boundaries and the real publication sequence, and
docs/DEVELOPMENT.md for the checks that actually run, including the container
and CI version skew that makes "just run pytest" misleading.

The env examples drifted the same way. ALLOWED_ORIGINS is a JSON array, not a
comma-separated list; the frontend needs FASTAPI_URL, DATABASE_URL and
PAYLOAD_SECRET, none of which were documented; and RATE_LIMIT_BURST,
DEFAULT_PAGE_SIZE and MAX_PAGE_SIZE were presented as tuning controls the routes
do not consult. Each is now stated as it behaves.

MIGRATION_SUMMARY.md keeps its content but gains a banner, because it reads like
setup instructions and is not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016y2J6bs8gbuSJbH18w7Tan

.env.example

@@ -20,7 +20,7 @@ PORT=80
 # =============================================================================
 # CORS
 # =============================================================================
-# Comma-separated list of allowed origins
+# JSON array of allowed origins (pydantic-settings format)
 # In production, only include your actual domain
 ALLOWED_ORIGINS=["https://schoolcompare.co.uk"]
 
@@ -33,13 +33,21 @@ ADMIN_API_KEY=CHANGE_THIS_TO_A_SECURE_RANDOM_KEY
 
 # Rate limiting (requests per minute per IP)
 RATE_LIMIT_PER_MINUTE=60
-RATE_LIMIT_BURST=10
+GLOBAL_RATE_LIMIT_PER_MINUTE=3000
 
 # Maximum request body size in bytes (default 1MB)
 MAX_REQUEST_SIZE=1048576
 
 # =============================================================================
-# API
+# SEARCH AND OPTIONAL FEATURE FLAGS
 # =============================================================================
-DEFAULT_PAGE_SIZE=50
-MAX_PAGE_SIZE=100
+TYPESENSE_URL=http://localhost:8108
+TYPESENSE_API_KEY=CHANGE_THIS_TO_YOUR_TYPESENSE_KEY
+
+# Empty URL disables Unleash-backed flags. Match the managed environment when used.
+UNLEASH_URL=
+UNLEASH_API_TOKEN=
+
+# Page-size limits are currently declared by route Query parameters.
+# DEFAULT_PAGE_SIZE, MAX_PAGE_SIZE and RATE_LIMIT_BURST are not reliable tuning
+# controls in the current routes; see docs/LEGACY_CODE.md.