feat: auto-detect backend from environment variables in DefaultBackendProvider

[27Bslash6]

Problem

DefaultBackendProvider always resolves to Redis regardless of which environment variables are set. Setting CACHEKIT_API_KEY has no effect unless you explicitly use @cache.io(), set_default_backend(CachekitIOBackend()), or pass backend=CachekitIOBackend().

This contradicts the "zero config" principle — a user who sets CACHEKIT_API_KEY clearly wants CachekitIO, but @cache.production silently ignores it and tries to connect to Redis.

Current Behavior

import os
os.environ["CACHEKIT_API_KEY"] = "ck_live_..."

@cache.production(ttl=300)
def get_product(product_id: str) -> dict: ...
# ❌ Tries to connect to Redis, not CachekitIO

Proposed Behavior

DefaultBackendProvider.get_backend() should resolve based on env var presence, with this priority:

1. CACHEKIT_API_KEY set → CachekitIOBackend (managed service gets priority)
2. CACHEKIT_REDIS_URL or REDIS_URL set → RedisBackend
3. CACHEKIT_MEMCACHED_SERVERS set → MemcachedBackend
4. Nothing set → L1 only (no backend, current zero-config behavior)

import os
os.environ["CACHEKIT_API_KEY"] = "ck_live_..."

@cache.production(ttl=300)
def get_product(product_id: str) -> dict: ...
# ✅ Auto-detects CachekitIO from CACHEKIT_API_KEY

Edge Cases

• Both CACHEKIT_API_KEY and CACHEKIT_REDIS_URL set: CachekitIO wins (priority 1). Log a warning noting both are configured.
• @cache.io() with no CACHEKIT_API_KEY: Should still raise ConfigurationError immediately (fail-fast, current behavior).
• Explicit backend= parameter: Always takes precedence over auto-detection (current behavior, unchanged).
• set_default_backend() called: Always takes precedence over auto-detection (current behavior, unchanged).

Impact

• @cache.io() becomes a convenience alias that validates the key upfront, rather than the only way to use CachekitIO
• All presets (@cache.production, @cache.secure, @cache.minimal, etc.) become genuinely backend-agnostic
• Docs can honestly say "switch backends by changing an environment variable"

Files

• src/cachekit/backends/provider.py — DefaultBackendProvider.get_backend()
• src/cachekit/cache_handler.py — get_backend_provider()
• Tests for priority resolution and edge cases

Context

Found while writing docs for docs.cachekit.io — the "Switching Backends" section originally claimed you could switch by swapping env vars, but that's not how DefaultBackendProvider works today.