fix(analysis): guardrails against model repetition loops

Small vision models (qwen3-vl-4b) sometimes loop the OCR array, running
the response into the token ceiling (finish_reason: length) and failing to
parse. Layered guardrails:

- Prompts: explicit "transcribe each element once, never repeat/loop, at
  most N, STOP when done" in the OCR, grounding and combined prompts.
- Schema: hard maxItems on ocr_results/tagging_results/content_type and
  maxLength on text/scene — LM Studio's grammar enforces these, so a loop
  is grammar-bounded rather than relying on max_tokens.
- Sampling: a configurable frequency_penalty (ANALYSIS_FREQUENCY_PENALTY,
  default 0.3) sent with each request — the decode-time lever that actually
  breaks loops.
- Code: sanitize() collapses runaway consecutive duplicates (same
  normalized text + kind) so a loop that slips through still can't flood
  the row.

Tests: schema caps present, frequency_penalty included only when nonzero,
sanitize collapses consecutive repeats; config default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/config.rs

@@ -141,6 +141,11 @@ pub struct AnalysisConfig {
     /// Output-constraint mode (`ANALYSIS_RESPONSE_FORMAT`):
     /// `json_schema` (default) | `json_object` | `none`.
     pub response_format: ResponseFormat,
+    /// Sampling `frequency_penalty` sent with each request
+    /// (`ANALYSIS_FREQUENCY_PENALTY`). A small positive value discourages
+    /// the repetition loops that otherwise run small models into the token
+    /// ceiling. `0` omits the field.
+    pub frequency_penalty: f64,
 }
 
 impl Default for AnalysisConfig {
@@ -164,6 +169,7 @@ impl Default for AnalysisConfig {
             max_slices: 16,
             max_image_bytes: 8 * 1024 * 1024,
             response_format: ResponseFormat::JsonSchema,
+            frequency_penalty: 0.3,
         }
     }
 }
@@ -199,6 +205,7 @@ impl AnalysisConfig {
             response_format: std::env::var("ANALYSIS_RESPONSE_FORMAT")
                 .map(|s| ResponseFormat::from_str(&s))
                 .unwrap_or(d.response_format),
+            frequency_penalty: env_f64("ANALYSIS_FREQUENCY_PENALTY", d.frequency_penalty),
         }
     }
 }
@@ -619,6 +626,7 @@ mod tests {
             "ANALYSIS_TALL_ASPECT",
             "ANALYSIS_MAX_SLICES",
             "ANALYSIS_RESPONSE_FORMAT",
+            "ANALYSIS_FREQUENCY_PENALTY",
         ] {
             std::env::remove_var(k);
         }
@@ -629,6 +637,7 @@ mod tests {
         assert_eq!(cfg.min_slice_height, 640);
         assert_eq!(cfg.max_slices, 16);
         assert_eq!(cfg.tall_aspect_threshold, 1.6);
+        assert_eq!(cfg.frequency_penalty, 0.3);
         assert!(cfg.api_key.is_none());
         assert_eq!(cfg.response_format, ResponseFormat::JsonSchema);
     }