Add user-configurable Hamming threshold

- Add PHASH_HAMMING_THRESHOLD environment variable to config with default of 3
- Update image processor to use config value instead of hardcoded 5
- Lower default hammingThreshold in rules from 5 to 3 for stricter matching
- Add Hamming threshold guide to README for reference

This allows users to control the global Hamming distance threshold via
environment variable while still allowing per-rule overrides. The lower
default of 3 reduces false positives from images that are too different.

README.md

···
 ];
 ```
 
+Hamming threshold guide:
+- 0 = Exact match only (very strict)
+- 1-2 = Nearly identical images (minor compression artifacts)
+- 3-4 = Very similar images (slight edits, crops)
+- 5-8 = Similar images (moderate edits)
+- 10+ = Loosely similar images (too permissive)
+
 To generate a phash from an image:
 
 ```bash

rules/blobs.ts

···
 
 export const BLOB_CHECKS: BlobCheck[] = [
   {
-    phashes: ["e0e0e0e0e0fcfefe"],
+    phashes: ["e0e0e0e0e0fcfefe", "9b9e00008f8fffff", "f090889c9ede9ee6"],
     label: "troll",
     comment: "Image is used in harrassment campaign targeting Will Stancil",
     reportAcct: false,
     labelAcct: true,
     reportPost: true,
     toLabel: true,
-    hammingThreshold: 5,
+    hammingThreshold: 3,
     description: "Sample harassment image variants (placeholder hashes)",
     ignoreDID: ["did:plc:7umvpuxe2vbrc3zrzuquzniu"],
   },
···
     labelAcct: false,
     reportPost: false,
     toLabel: true,
-    hammingThreshold: 5,
+    hammingThreshold: 3,
     description: "Sample harassment image variants (placeholder hashes)",
   },
 ];

src/config/index.ts

···
     modDid: string;
     rateLimit: number;
   };
+  phash: {
+    defaultHammingThreshold: number;
+  };
 }
 
 function getEnv(key: string, defaultValue?: string): string {
···
   moderation: {
     modDid: getEnv("MOD_DID"),
     rateLimit: getEnvNumber("RATE_LIMIT_MS", 100),
+  },
+  phash: {
+    defaultHammingThreshold: getEnvNumber("PHASH_HAMMING_THRESHOLD", 3),
   },
 };

src/processor/image-processor.ts

···
         n &= n - 1n;
       }
 
-      const threshold = matchedCheck.hammingThreshold ?? 5;
+      const threshold = matchedCheck.hammingThreshold ?? config.phash.defaultHammingThreshold;
       if (distance <= threshold) {
         return {
           phash,