Make the README's storage, formats, TLS and metrics match the code (closes #74)

"Storage" names the cache directories pixa uses (cache/sources,
cache/metadata, cache/variants) and how files are named in each; the
schema comments name the same paths, and the output_content comment says
the table is not written. Routes and the signature section give one list
of output formats, jpg and original included, and say how those two are
signed. The TLS sentence names allow_http as its exception. "Metrics"
says what exists: generic HTTP and Go runtime metrics at /metrics,
measured and served only when the metrics username and password are set.

Model: opus-5-5

internal/db/migrations/001_schema.sql

@@ -2,7 +2,7 @@
 -- Creates all tables for the pixa caching image proxy
 
 -- Source content blobs
--- Files stored at: cache/src-content/<ab>/<cd>/<sha256>
+-- Files stored at: cache/sources/<ab>/<cd>/<sha256>
 -- last_accessed_at is NULL until the first LRU touch; eviction falls
 -- back to fetched_at for rows that have never been touched.
 CREATE TABLE IF NOT EXISTS source_content (
@@ -16,7 +16,7 @@ CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
     ON source_content(last_accessed_at);
 
 -- Source URL metadata - maps URLs to content hashes
--- JSON stored at: cache/src-metadata/<hostname>/<path_hash>.json
+-- JSON stored at: cache/metadata/<hostname>/<path_hash>.json
 CREATE TABLE IF NOT EXISTS source_metadata (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     source_host TEXT NOT NULL,
@@ -56,7 +56,8 @@ CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
     ON variant_content(last_accessed_at);
 
 -- Output/transformed content blobs
--- Files stored at: cache/dst-content/<ab>/<cd>/<sha256>
+-- Not written: transformed images are stored in cache/variants and
+-- tracked in variant_content above.
 CREATE TABLE IF NOT EXISTS output_content (
     content_hash TEXT PRIMARY KEY,
     content_type TEXT NOT NULL,