Enforce and document exact-match-only for signature verification (#40)

Closes #27

Signatures are per-URL only — this PR adds explicit tests and documentation enforcing that HMAC-SHA256 signatures verify against exact URLs only. No suffix matching, wildcard matching, or partial matching is supported.

## What this does NOT touch

**The host whitelist code (`whitelist.go`) is not modified.** This PR is exclusively about signature verification, per sneak's instructions on [issue #27](#27), [PR #32](#32), and [PR #35](#35).

## Changes

### `internal/imgcache/signature.go`
- Added documentation comments on `Verify()` and `buildSignatureData()` explicitly specifying that signatures are exact-match only — no suffix, wildcard, or partial matching

### `internal/imgcache/signature_test.go`
- **`TestSigner_Verify_ExactMatchOnly`**: 14 tamper cases verifying that modifying any signed component (host, path, query, dimensions, format) causes verification to fail. Host-specific cases include:
  - Parent domain (`example.com`) does not match subdomain signature (`cdn.example.com`)
  - Sibling subdomain (`images.example.com`) does not match
  - Deeper subdomain (`images.cdn.example.com`) does not match
  - Evil suffix domain (`cdn.example.com.evil.com`) does not match
  - Prefixed host (`evilcdn.example.com`) does not match
- **`TestSigner_Sign_ExactHostInData`**: Verifies that suffix-related hosts (`cdn.example.com`, `example.com`, `images.example.com`, etc.) all produce distinct signatures

### `internal/imgcache/service_test.go`
- **`TestService_ValidateRequest_SignatureExactHostMatch`**: Integration test through `ValidateRequest` verifying that a valid signature for `cdn.example.com` is rejected when presented with a different host (parent domain, sibling subdomain, deeper subdomain, evil suffix, prefixed host)

### `README.md`
- Updated Signature Specification section to explicitly document exact-match-only semantics

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #40
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

README.md

@@ -67,7 +67,10 @@ hosts require an HMAC-SHA256 signature.
 #### Signature Specification
 
 Signatures use HMAC-SHA256 and include an expiration timestamp to
-prevent replay attacks.
+prevent replay attacks. Signatures are **exact match only**: every
+component (host, path, query, dimensions, format, expiration) must
+match exactly what was signed. No suffix matching, wildcard matching,
+or partial matching is supported.
 
 **Signed data format** (colon-separated):